Kepler Mission - Hunting for Exoplanets
Kepler is part of NASA's Discovery Program designed to survey a portion of our region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets. The primary goal is to determine the frequency of Earth-size and larger planets in the HZ (Habitable Zone) of solar-like stars. The mission will monitor more than 100,000 stars for patterns of transits with a differential photometric precision of 20 ppm at V = 12 for a 6.5 hour transit. It will also provide asteroseismic results on several thousand dwarf stars. It is specifically designed to continuously observe a single FOV (Field of View) of > 100 deg 2 for 3.5 or more years. 1)
Finding extrasolar planets is extremely challenging and was not accomplished until 1995 when Mayor & Queloz, (1995) detected the first jovian-mass planet around normal stars. However, by making the observations from a spaceborne platform and using the transit method proposed by Borucki and Summers (1984), Earth-size planets, including those in the HZ, should be detected in substantial numbers.
The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:
• Determine the percentage of terrestrial and larger planets that are in or near the habitable zone of a wide variety of stars
• Determine the distribution of sizes and shapes of the orbits of these planets
• Estimate how many planets there are in multiple-star systems
• Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
• Identify additional members of each discovered planetary system using other techniques
• Determine the properties of those stars that harbor planetary systems.
The Kepler Mission also supports the objectives of future NASA Origins theme missions SIM (Space Interferometry Mission) and TPF (Terrestrial Planet Finder),
• By identifying the common stellar characteristics of host stars for future planet searches
• By defining the volume of space needed for the search and
• By allowing SIM to target systems already known to have terrestrial planets.
Background of the Kepler Mission: 2)
The Kepler Mission developed over several decades as a way of answering the question: How frequent are other Earths in our galaxy? In particular, what is the frequency of Earth-size planets in the HZ (Habitable Zone) of solar-like stars? In the last half of the twentieth century, the astrometric and interferometric approaches to finding exoplanets were the favored methods. The surprising discovery by Wolszczan (1994) 3) based on timing of radio pulses from pulsars showed that a wider range of approaches should be considered. A paper by Rosenblatt (1971) 4) provided a quantitative discussion of another alternative method; searching for patterns of transits to get size and orbital period. To be successful, all three approaches depend upon adapting new technology; the underlying principles are well understood. A paper by Borucki and Summers (Ref. 3) corrected the detection probability in the paper and pointed out that ground-based observations of at least 13,000 stars simultaneously should be sufficient to detect Jovian-size planets, but that the detection of Earth-size planets would require space-based observations. Limitations to the detectability of planets by stellar variations was recognized (Borucki et al, 1985) 5) and discussed more fully by Jenkins (2002). 6)
To examine the technology needed to accomplish transit detection of exoplanets, NASA/ARC ( Ames Research Center) sponsored a workshop on high precision photometry in 1984 (Proceedings of the Workshop on Improvements to Photometry, 1984). The success of the first workshop encouraged a second workshop (Second Workshop on Improvements to Photometry, 1988) jointly sponsored by ARC and the NBS (National Bureau of Standards, now NIST) at Gaithersburg, Maryland in 1987. A wide range of subsystems was discussed including very stable band pass filters, 16-bit analog to digital converters, electronic amplifiers, and detectors. Ion-beam bombardment of band pass filters and the use of silicon diodes were recommended.
To further develop this approach NASA HQ funded the development and testing of proof-of-concept multichannel photometers based on silicon photodiodes. Tests conducted at the NBS and at Ames showed that the diodes had very high precision as expected, but that to reduce their thermal noise, they would need to be cooled to near liquid nitrogen temperatures. Two cooled multichannel photometers were built; the latter was based on an optical fiber feed to the cooled diodes. Erratic transmission of the multimode fibers doomed the latter (Borucki et al.1987 and Borucki et al.1988). 7) 8)
In 1992, NASA HQ proposed a new line of missions to address questions about the Solar System and that would also consider the search for exoplanets. Proposals for concept studies were invited and discussed at a workshop at San Juan Capistrano, CA. The proposals were for complete missions: science, technical, engineering, management, cost, and schedule were to be addressed. For this opportunity, a team was organized to propose a transit search for terrestrial planets. The proposed mission was called FRESIP (FRequency of Earth-Size Inner Planets) to describe its goal.
The review panel found that the science value was very high and would have supported the concept had there been proof that detectors existed with sufficient precision and the requisite low noise to find Earth-size planets.
In 1994, the first flight opportunity for a Discovery-Class mission was announced. FRESIP proposed a 0.95 m aperture photometer to be placed in a Lagrange orbit. CCD detectors were substituted for the silicon detectors because of their capability of tracking many targets simultaneously and their ability to accept many different target patterns. The review panel considered the FRESIP photometer to be a telescope similar to the HST (Hubble Space Telescope) and thus far too expensive to qualify as a Discovery-class mission. The proposal was rejected.
Lab tests to prove that CCD detectors were suitable were funded by small grants from NASA HQ and ARC. The first paper presenting the results of lab tests demonstrating the CCD detectors had the requisite precision and low noise to detect transit patterns of Earth-size transits was published in 1995 (Robinson et al. 1995) 9). The experiment was carried out in the basement of Lick Observatory and used an old 512 x 512 Reticon front-side illuminated CCD. For many of the simulated stars a precision of 5 x 10 -6 was achieved. Back-side illuminated CCDs, where the light does not pass through the wire traces on its way to the active silicon were expected to have even higher precision. An accidental spilling of liquid nitrogen during the lab tests did not cause loss of precision because the records of centroid movement allowed the motions to be regressed out. In fact it was the mathematical identification and removal of the systematic noise that was the break through step that allowed the intrinsic precision of these detectors to be recognized.
In 1996, the second opportunity to propose for a flight mission was announced. Studies showed that mission costs could be reduced if photometer was placed in a solar orbit rather than a Lagrange orbit because of the reduction of space propulsion systems needed to stay in a Lagrange orbit. At the insistence of several members (Koch, Tarter, Sagan) of the team, the mission name was changed from FRESIP to "Kepler" to honor the German astronomer (Johannes Kepler, 1571-1630) who developed the laws of planetary motion and the principle needed to calculate optical prescriptions. Both are critical to the operation of the current mission. The mission cost was estimated in three different ways to show that the mission cost could be accomplished for the available budget. The proposal was rejected because no one had every demonstrated that the simultaneous, automated photometry of thousands of stars could be done. The review panel recommended that we build such a photometer to demonstrate the methods to be used. Funding was granted for such a demonstration from both NASA/HQ and NASA/ARC.
In 2000, the fourth opportunity to propose for a Discovery-class mission was announced and Kepler proposed for the fifth time. Kepler was one of three proposals selected from a total of 26 that was allowed to compete by writing a Concept Study Report and demonstrating readiness to proceed.
In December of 2001, Kepler was selected as Discovery Mission #10. Mission development started in 2002 by placing orders for the detectors.
During the years prior to selection, many events helped get the Mission concept accepted. Two major events were the discovery of extrasolar planets by Michel Mayor's team (Mayor and Queloz 1995)10) and Geoff Marcy's team (Marcy and Butler, 1996) 11) and success by several ground based transit search groups (Charbonneau et al. 2000). 12) Once the radial velocity technique had convincingly demonstrated that many exoplanets existed and NASA HQ recognized that the transit technique was proven and that the technology existed that could find Earth-size planets, both the development of the Kepler Mission and a vigorous ground based efforts were funded. In particular, the many years that the Kepler team devoted to convincing the science community, the technical review panels, and NASA HQ officials, helped promote the funding of ground-based transit surveys that are now so successful in finding and characterizing exoplanets. In turn the success of both the radial velocity and transit approaches helped the Kepler Mission to compete against the many excellent proposals received at every AO for a Discovery-class mission.
When a planet crosses in front of its star as viewed by an observer, the event is called a transit (Figure 1). Transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, ppm), lasting for 1 to 16 hours. This change must be periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.
Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. Knowing the temperature of a planet is key to whether or not the planet is habitable (not necessarily inhabited). Only planets with moderate temperatures are habitable for life similar to that found on Earth.
Target FOV (Field of View): Since transits only last a fraction of a day, all the stars must be monitored continuously, that is, their brightnesses must be measured at least once every few hours. The ability to continuously view the stars being monitored dictates that the FOV must never be blocked at any time during the year. Therefore, to avoid the Sun the FOV must be out of the ecliptic plane. The secondary requirement is that the FOV have the largest possible number of stars. This leads to the selection of a region in the Cygnus and Lyra constellations of our Galaxy as shown.
Figure 1: Illustration of a transit by a planet in front of its star as viewed by an observer (image credit: NASA/ARC)
Figure 2: Kepler's Field Of View In Targeted Star Field (image credit: NASA/ARC)
Kepler Science Team:
Hundreds of people across the country are involved in the Kepler Mission. NASA/JPL (Jet Propulsion Laboratory), Pasadena, Calif., managed the development of the project for NASA/ARC (Ames Research Center), Moffett Field, Calif., and is responsible for ensuring that Kepler's flight system performs successfully on orbit. NASA Ames managed the development of the ground system and will conduct scientific analysis for the mission. BATC (Ball Aerospace and Technologies Corporation) developed Kepler's flight system, including the spacecraft and the photometer, and is participating in mission operations. NASA Ames will manage flight operations after commissioning is completed (Ref.19) .
The Science Principal Investigator is William Borucki and the Deputy Principal Investigator is David Koch, both of NASA's Ames Research Center. Other members of Kepler's science team include Co-Investigators, a science working group and participating scientists.
The Co-Investigators include Gibor Basri, University of California at Berkeley, Berkeley, Calif.; Natalie Batalha, San Jose State University, San Jose, CA; Timothy Brown, LCOGT (Las Cumbres Observatory Global Telescope), Goleta, CA; Doug Caldwell, SETI Institute, Mountain View, CA; Jørgen Christensen-Dalsgaard, University of Aarhus, Denmark; William Cochran, McDonald Observatory, University of Texas at Austin; Edna DeVore, SETI Institute; Edward Dunham, Lowell Observatory, Flagstaff AZ; Nick Gautier, JPL, Pasadena, CA; John Geary, SAO (Smithsonian Astrophysical Observatory), Cambridge, MA; Ronald Gilliland, STScI (Space Telescope Science Institute), Baltimore, MD; Alan Gould, LHS (Lawrence Hall of Science), Berkeley, CA; Jon Jenkins, SETI Institute; Yoji Kondo, NASA/GSFC (Goddard Space Flight Center), Greenbelt, MD; David Latham, SAO; Jack Lissauer, NASA Ames; Geoff Marcy, University of California at Berkeley; David Monet, USNO (US Naval Observatory), Flagstaff Station, Flagstaff, AZ and Dimitar Sasselov, Harvard University, Cambridge, MA.
The Science Working Group is comprised of Alan Boss, Carnegie Institution of Washington, Washington D.C.; John J. Caldwell, York University, Canada; Andrea Dupree, SAO; Steve Howell, NOAO (National Optical Astronomy Observatory), Tucson, AZ; Hans Kjeldsen, University of Aarhus, Denmark; Soren Meibom, SAO; David Morrison, NASA Ames and Jill Tarter, SETI Institute.
Participating Scientists are Derek Buzasi, Eureka Scientific, Oakland, Calif.; Matt Holman, Harvard-Smithsonian CfA (Center for Astrophysics), Cambridge, MA; David Charbonneau, CfA; Sara Seager, Massachusetts Institute of Technology, Cambridge, MA; Laurance Doyle, SETI Institute; Jason Steffen, Fermi National Accelerator Laboratory, Batavia, Ill; Eric Ford, University of Florida, Gainsville; William Welsh, San Diego State University, San Diego, CA and Jonathan Fortney, University of California at Santa Cruz, Santa Cruz, CA.
The team members collaborate on various tasks within the project. For example:
• Scientists at SAO, USNO and LCOGT made the observations and interpreted the data used to build the Kepler Input Catalog.
• Scientists at SAO, Harvard, University of California at Berkeley, University of Texas at Austin, NOAO, Lowell Observatory and JPL will conduct the follow-up observing work to confirm discoveries, detect other planets in the systems and improve our understanding of the stellar properties.
• Educators at LHS and SETI Institute conduct the Education and Public Outreach program.
• Scientists at the University of Aarhus lead the Kepler Asteroseismic Science Consortium that determines stellar masses, sizes and ages from the Kepler data.
The Kepler Space Observatory, a PI (Principal Investigator) class mission, was competitively selected as NASA's tenth Discovery mission. NASA selected BATC (Ball Aerospace and Technologies Corporation) of Boulder, CO, as the prime contractor for both the photometer and spacecraft. The prime contractor is also responsible for operating the mission. This approach removes many contractual barriers to optimal mission design, efficiency, risk, and schedule for the flight hardware and software. Having a single contractor allows for a single systems engineering team and common subsystem engineering teams for software, thermal, integration and test, etc. for both the photometer and the spacecraft. This approach has allowed for the broadest possible trade space when conducting studies and further eliminates the need for defining many controlled interfaces to external entities, which may often be artificial. 15) 16) 17)
Systems engineering is an important discipline in the development and execution of space-astronomy missions. As observatories and instruments grow in size, complexity, and capability, we are forced to deal with new performance regimes – in many cases forcing us to find solutions to issues and error sources that could be safely ignored on past missions. Systems engineering, if applied rigorously and judiciously, can bring to bear a suite of processes and tools that can help balance risk, cost, and mission success. 18)
The Kepler mission has been optimized to search for Earth-size planets (0.5 to 10 earth masses) in the HZ (Habitable Zone) of solar-like stars. Given this design, the mission will be capable of not only detecting Earth analogs, but a wide range of planetary types and characteristics ranging from Mars-size objects and orbital periods of days to gas-giants and decade long orbits. The mission is designed to survey the full range of spectral-types of dwarf stars. Kepler utilizes photometry to detect planet's transiting their parent star. Three or more transits of a star with a statistically consistent period, brightness change and duration provide a rigorous method of detection. From the relative brightness change the planet size can be calculated. From the period the orbital size can be calculated and its location relative to the HZ determined.
The Kepler spacecraft (Figure 3) has significant heritage from Deep Impact and Orbital Express for many of its subsystems, particularly the avionics. The purpose of the spacecraft is to provide power, pointing and telemetry for the photometer. The three-axis-stabilized spacecraft is fully redundant and single-fault tolerant.
ADCS (Attitude Determination and Control Subsystem): Of primary concern for achieving the photometric precision is attitude stability. Image motion has an adverse affect on the photometric precision due to both the extended wings of the psf and the inter- and intra-pixel responsivity variations. The requirement is to keep the temporal frequency of anything that can affect the photometric precision well outside of the time domain for a transit. Transits can occur on time scales from an hour or so (a grazing transit of a planet with an orbit of a few days) up to 16 hours (a central transit of a planet with an orbit like Mars). To achieve the short term stability the ADCS needs to operate at about 10 Hz to keep jitter low. The specification is 0.1 arcsec (3σ) about each of three axes. To prevent long-term drifts, four fine guidance sensor CCDs are mounted to the scientific focal plane at the four corners. Note that in heliocentric orbit, the only external torque is solar radiation pressure (photons). Unlike Earth orbit, there is no gravity gradient, magnetic torquing or atmospheric drag. Control is provided by four reaction wheels, which are unloaded periodically by a twelve-thruster hydrazine reaction control system. There are ten coarse sun sensors, two star trackers, and two three-axes inertial measurement units for initial acquisition, roll maneuvers and safe-survival modes.
Figure 3: The Kepler spacecraft with the HGA (High Gain Antenna) deployed and the photometer (image credit: NASA, KeplerTeam)
EPS (Electrical Power Subsystem): The EPS is based on a direct-energy transfer architecture. The solar array is designed to produce at least 615 W at 29±4 V at the end of mission in the nominal observing attitude. Solar-array strings are switched as required to provide power to flight segment loads. The spacecraft is rotated 90º every three months to maintain the Sun on the solar array. The solar array is thermally isolated from the spacecraft and photometer. A Li-ion battery is provided to support launch and emergency modes, but is not needed for the observing mode.
The solar array is rigidly mounted to the spacecraft's upper deck. As such, it pulls double-duty on this mission, providing power, as well as shielding the photometer from direct solar heating. The solar array is on four non-coplanar panels and totals 10.2 m2 of triple-junction photovoltaic cells. It contains 130 strings each composed of 22 cells. The solar array is expected to generate up to 1,100 W of electrical power. Unlike most spacecraft solar arrays that are deployed or articulated, Kepler's solar array is fixed.
TCS (Thermal Control Subsystem): TCS is responsible for maintaining spacecraft component temperatures within operational limits. The solar array and thermal blankets shield the photometer from direct solar heating. The solar panels themselves are made out of a special material to minimize heat flow to the photometer, and their finishes also help regulate panel temperature. Kepler is also protected by an "active" thermal control system that consists of heat pipes, thermally conductive adhesives, heaters and temperature sensors. Propane and ammonia flowing through pipes embedded in the spacecraft's exterior panels cool the focal plane. Various parts of the spacecraft that need to be heated in order to operate are equipped with controlled heaters but insulated to avoid heating the photometer.
Avionics: The spacecraft avionics are derived from the design used for the Orbital Express mission. They are fully redundant and can be cross switched between the A and B sides. The processors are the same as for the photometer, radiation hardened PowerPC 750s built by BAE. The avionics provide command and telemetry processing, formatting and storage of spacecraft housekeeping data, thermal control processing, ADCS processing, a mission unique board for items like the cover release, and network interfaces between all of the subsystems and with the photometer. Redundant crystal oscillators are used for on-board time keeping with drift rates of less than 5 x 10-11.
RF Telecommunications: Telemetry for the stored data will be transmitted to the ground using a Ka-band (32 GHz) high-gain antenna (HGA) with a diameter of 0.8 m. Data rates range up to 2.88 Mbps and use a 35 W TWTA (Traveling Wave Tube Amplifier). The command uplink and realtime engineering data downlink will use an omni-directional X-band (8 GHz) antenna system and a 25 W TWTA. A 34 m BWG (Beam Wave Guide) antenna is baselined for the uplink transmitter. The one-time release HGA boom and the redundant two-axes gimbal are the only mechanisms on the spacecraft. The command contacts and data downlinks should not interrupt the precision or recording of the scientific data.
Table 1: Key spacecraft parameters 19)
Spacecraft structures and mechanisms: The majority of Kepler's systems and subsystems are mounted on a low-profile hexagonal box which is wrapped around the base of the photometer. The hexagonal box structure consists of six shear panels, a top deck, bottom deck, reaction control system deck, and the launch vehicle adapter ring. Construction of the shear panels, decks, and solar array substrates, consists of sandwiched aluminum face-sheets on an aluminum honeycomb core. The six shear panels provide structure to accommodate mounting of the spacecraft electronics, portions of the photometer electronics, battery, star trackers, reaction wheels, inertial measurement units, radio equipment, and high- and low-gain antennas.
The top deck shear panel provides the mounting surface for the solar array panels. The bottom deck provides the interface to the photometer and also supports the thrusters, associated propellant lines, and launch vehicle umbilical connectors. The reaction control system deck is attached to the inside of the launch vehicle adapter ring, and provides a mounting surface for the tank, pressure transducer, latch valves, and propellant lines. The base of the photometer is mounted to the lower deck.
Figure 4: Kepler spacecraft integrated with Photometer (image credit: NASA, Kepler Team)
Figure 5: The Kepler spacecraft in Astrotech's Hazardous Processing Facility in Titusville, FL in February 2009 (image credit: NASA)
Launch: The Kepler Observatory was launched on March 7, 2009 (03:49:57 UTC) on a ULA (United Launch Alliance) Delta-II 7925-10L vehicle from Space Launch Complex 17B at the Cape Canaveral Air Force Station, FL. 20)
Orbit: The continuous viewing needed for a high detection efficiency for planetary transits requires that theFOV (Field of View) of the photometer be out of the ecliptic plane so as not to be blocked periodically by the Sun or the Moon. A star field near the galactic plane that meets these viewing constraints and has a sufficiently high star density has been selected. 21)
An Earth-trailing heliocentric orbit with a period of 372.5 days provides the optimum approach to meeting of the combined Sun-Earth-Moon avoidance criteria within the launch vehicle capability. In this orbit the spacecraft slowly drifts away from the Earth and is at a distance of 0.5 AU (worst case) at the end of four years. Telecommunications and navigation for the mission are provided by NASA's DSN (Deep Space Network).
Figure 6: The spacecraft must execute a 90 degree roll every 3 months to reposition the solar panels to face the Sun while keeping the instrument aimed at the target field of view (image credit: NASA)
Figure 7: Illustration of the Kepler spacecraft in orbit (image credit: NASA)
Key Mission Requirements:
Key considerations when looking for planetary transits are the probability for the orbital plane to be aligned along the line of sight and the number of stars to monitor. The probability of orbital alignment is simply the ratio of the stellar diameter to the orbital diameter. For the Sun-Earth analogy the probability is 0.5%. Hence, one needs to monitor many thousands of stars before one can arrive at a statistically meaningful result, null or otherwise.
In addition, a sequence of transits with a consistent period, depth and duration must be detected to be confident and to confirm the existence of a planet. A Sun-Earth-like transit produces an apparent change in brightness of the star of 84 ppm (parts per million) with a duration of 13 hours, if it crosses near the center of the star. For a statistically significant detection, the minimum single transit Signal to Noise Ratio (SNR) requirement is taken to be 4σ, leading to a combined average significance of 8σ for 4 such transits. The detection threshold is set at 7σ, yielding a detection rate of 84% while controlling the total number of expected false alarms to no more than one for the entire experiment. The total system noise, defined to be the CDPP (Combined Differential Photometric Precision), must be less than 21 ppm in 6.5 hours (half of a central transit duration).
The resulting driving requirements for the Kepler Mission are:
1) A CDPP of 20 ppm in 6.5 hrs and the ability to detect a single Earth-like transit with an SNR>4
2) The capability to monitor >100,000 stars simultaneously (>170,000 stars in the first year)
3) A mission duration of at least four years.
Sensor complement: (Photometer)
Table 2: Main parameters of the photometer
The instrument has the sensitivity to detect an Earth-size transit of an mv=12 G2V (solar-like) star at 4 σ in 6.5 hours of integration. The instrument has a spectral bandpass from 400 nm to 850 nm. Data from the individual pixels that make up each star of the 100,000 main-sequence stars brighter than mv=14 are recorded continuously and simultaneously. The data are stored on the spacecraft and transmitted to the ground about once a month. 22)
The sole instrument aboard Kepler is a photometer (or light meter), an instrument that measures the brightness variations of stars. The photometer consists of the telescope, the focal plane array and the local detector electronics.
Telescope: Kepler has a very large field of view — approximately 100 square degrees — for an astronomical telescope. The photometer optics are a modification of the classic Schmidt telescope design. They include a 0.95 m aperture fused-silica Schmidt corrector plate and a 1.4 m diameter 85% light weighted ultra-low expansion-glass primary mirror. The mirror has an enhanced silver coating. The optical design results in 95% of the energy from a star being distributed over an area at the focal plane of approximately seven pixels in diameter. The primary mirror is mounted onto three focus mechanisms, which may be used in flight to make fine focus adjustments. The focus mechanisms can adjust the mirror's piston, tip and tilt. While electrical power is required to move the focus mechanisms, they are designed to hold the position of the primary mirror without continuous power. A sunshade is mounted at the front of the telescope to prevent sunlight from entering the photometer. Kepler is the ninth largest Schmidt telescope ever built and the largest telescope ever to be launched beyond Earth orbit.
Figure 9: Inspection of the 1.4 meter primary mirror honeycomb structure. The mirror has been 86% light weighted, and only weighs 14% of a solid mirror of the same dimensions (image credit: NASA, Kepler Team)
FPA (Focal Plane Array): At the heart of the photometer is the Focal Plane Array. This consists of a set of CCDs (Charged Coupled Devices), sapphire field flattening lenses, an invar substrate, heat pipes and radiator.
The CCDs are the silicon light-sensitive chips that are used in today's TV cameras, camcorders and digital cameras. The CCDs aboard Kepler are not used to take pictures in the conventional sense. Kepler's wide-field optics reflect light from the star field onto the array of 42 CCDs. Each of the 42 CCDs are 59 x 28 mm in size and contain 2,200 by 1024 pixels, that is, individual picture elements, for a total of 95 Mpixels. The CCDs are four-phase, thinned, back-illuminated and anti-reflection coated devices. Each device has two outputs, resulting in a total of 84 data channels. The CCDs are mounted in pairs and have a single sapphire field-flattening lens over each pair. The optics spread the light of the stars over several pixels within an individual CCD to improve differential photometry thus making the system less sensitive to inter-pixel response variations and pointing jitter.
The focal plane is cooled to about -85º Celsius by heat pipes that carry the heat to an external radiator. Data from the CCDs are extracted every six seconds to limit saturation and added on board to form a 30-minute sum for each pixel. The array is supported midway between the Schmidt corrector and the primary mirror.
Figure 10: Completed flight focal plane array with the 42 science CCDs and four fine guidance CCDs in the corners (image credit: NASA, Kepler Team)
Local detector electronics: A local detector electronics box communicates with the 84 data channels and converts the CCD output analog signals into digital data. The electronics box is located directly behind the focal-plane array in the center of the photometer structure. It has more than 22,000 electronic components tightly packed into a volume measuring slightly more than one cubic foot. Careful thermal engineering was required in order to isolate the cold detectors from the heat of the detector electronics. The data are stored in the spacecraft's solid-state recorder and transmitted to the ground approximately once a month.
Data handling: Since the entire 95 Mpixels of data cannot be stored continuously for 30 days, the science team has pre-selected the pixels of interest associated with each star of interest. This amounts to about 5 %of the pixels. These data are then requantized, compressed and stored. The on-board photometer flight software gathers the science and ancillary pixel data and stores them in a 16 GB solid-state recorder. Data are required to be stored and downlinked for science stars, p-mode stars, smear, black level, background and full FOV images.
The Kepler focal plane is approximately 30 x 30 cm in size. It is composed of 25 individually mounted modules. The 4 corner modules are used for fine guiding and the other 21 modules are used for science observing. Attached are some pictures that show a single science module and the assembled focal plane with all 25 modules installed.
Note that the fine guidance modules in the corners of the focal plane are very much smaller CCDs than the science modules. On the left, a single science module with two CCDs and a single field flattening lens mounted onto an Invar carrier. On the right of Figure 11, a focal plane assembly with all 21 science modules and four fine-guidance sensors, one in each corner, installed. Under normal operations, each module and its electronics convert light into digital numbers. For the darkest parts of the image between stars, we expect these numbers to be very small (but not zero). Correspondingly, for the brightest stars in the image, much larger numbers are expected creating an image of each observed star and its background neighborhood. 23)
Figure 11: Left: Single 2-CCD module, Right: Full focal plane CCD assembly (image credit: NASA, Kepler Team)
Selecting the Kepler Star Field: The star field for the Kepler Mission was selected based on the following constraints:
1) The field must be continuously viewable throughout the mission.
2) The field needs to be rich in stars similar to our sun because Kepler needs to observe more than 100,000 stars simultaneously.
3) The spacecraft and photometer, with its sunshade, must fit inside a standard Delta II launch vehicle.
The size of the optics and the space available for the sunshield require the center of the star field to be more than 55º above or below the path of the sun as the spacecraft orbits the sun each year trailing behind the Earth.
This left two portions of the sky to view, one each in the northern and southern sky. The Cygnus-Lyra region in the northern sky was chosen for its rich field of stars somewhat richer than a southern field. Consistent with this decision, all of the ground-based telescopes that support the Kepler team's follow-up observation work are located at northern latitudes.
Distances to the Kepler Stars: Kepler will be looking along the Orion spiral arm of our galaxy. The distance to most of the stars for which Earth-size planets can be detected by Kepler is from 600 to 3,000 light years. Less than 1% of the stars that Kepler will be looking at are closer than 600 light years. Stars farther than 3,000 light years are too faint for Kepler to observe the transits needed to detect Earth-size planets.
Figure 12: The Kepler Field of View (image credit: NASA, Kepler Team)
The ground segment facilities, shown in Figure 13, are used to operate the Flight Segment and analyze the data. Overall mission direction will be provided from the Mission Management and Science Offices hosted by the SOC ( Science Operations Center) at NASA/ARC ( Ames Research Center) in Mountain View, California. Strategic mission planning and target selection is done at the SOC. Target selection will utilize an input catalog especially generated by a team of Co-Is (Co-Investigators) led by the SAO (Smithsonian Astrophysical Observatory) that will provide the means to discriminate between dwarf and giant stars. 24)
Scientific data analysis, direction for the FOP (Follow-up Observing Program) implemented by Co-Is, and final interpretation will also be performed at NASA Ames. Flight Segment operations management, tactical mission planning, sequence validation, and engineering trend analysis will be directed by a FPC (Flight Planning Center) at BATC (Ball Aerospace Technologies Corporation) in Boulder, Colorado. Command and data processing, Flight Segment health & status monitoring, DSN scheduling, and spacecraft ephemeris propagation is the responsibility of the MOC (Mission Operations Center) at HTSI (Honeywell Technology Solutions Inc.) facility in Columbia, Maryland. Uplink and downlink telecommunications will use the NASA/JPL DSMS (Deep Space Mission System ), i.e., the DSN 34 m antennas located around the world.
The DMC (Data Management Center) at the STScI (Space Telescope Science Institute) in Baltimore, Maryland receives the "raw" Level 1 data and performs pixel-level calibration. The resulting Level 2 data set is archived by the DMC and forwarded to the SOC for further processing. The SOC processing includes generation of calibrated photometric light curves (returned to the DMC as a Level 3 data set for inclusion in the Kepler public archive) and transit detection. STScI also provides p-mode analysis. After an extensive data validation process, follow-up observations on each planetary candidate will be performed. The FOP is necessary to eliminate intrinsic false positives due to grazing-eclipsing binaries and extrinsic false positives due to background eclipsing binaries or discriminate between terrestrial transits of the target star and giant planet transits of a background star.
Figure 13: Overview of the ground segment (image credit: NASA, Kepler Team)
Systems Engineering Organization
While there are 6 major partners involved in the Kepler mission, the bulk of the systems engineering work at the Mission/Project System-level and Segment-levels is performed via collaborative effort between JPL, ARC, and BATC. The distribution of effort can be split into 4 major tasks: Science Systems Engineering, Mission/Project Systems Engineering, Flight Segment Systems Engineering, and Ground Segment Systems Engineering. Given the Kepler team structure, these 4 tasks are covered in a distributed fashion. For example, the Science Office at ARC leads the "Science Systems Engineering" effort – which primarily involves science requirements synthesis and follow-up observing program planning - and receives significant support from the Ground Segment Systems Engineer at ARC and the Project System Engineer and Mission Scientist at JPL in the areas of requirements sub-allocation and validation. Likewise, the Project System Engineer leads mission-level engineering efforts but receives substantial support from the Flight Segment System Engineer at BATC in the areas of end-to-end performance modeling, mission-level technical performance metric tracking, launch vehicle interface definition, and mission planning and trajectory design.
Given the potential for confusion and/or gaps in the lines of roles and responsibilities (an issue which seriously impacted some past missions), the Kepler project was careful to establish the diagram in Figure 14 which clarifies those relationships. It's worth highlighting the need for tight coupling of science and engineering on space missions. Care must be taken to avoid misunderstandings and gaps between science needs and engineering implementations. On Kepler, we have established a very tightly knit systems engineering team, which includes representation from the Science Team in the form of the Deputy PI and Mission Scientist – together with the Project System Engineer, they have "one foot rooted in each camp".
Figure 14: System Engineering Roles & Responsibilities (image credit: NASA, Kepler Team)
Kepler / K2 mission status:
• July 17,2018: To discover the presence of a planet around stars, astronomers wait until it has completed three orbits. However, this effective technique has its drawbacks since it cannot confirm the presence of planets at relatively long periods. To overcome this obstacle, a team of astronomers under the direction of the University of Geneva, Switzerland (UNIGE), have developed a method that makes it possible to ensure the presence of a planet in a few months, even if it takes 10 years to circle its star: this new method is described for the first time in the journal Astronomy & Astrophysics. 25) 26)
- The method of transits, consisting of detecting a dip in the luminosity of the host star at the time the planet passes, is a very effective technique to search for exoplanets. It makes it possible to estimate the radius of the planet, the inclination of the orbit and can be applied to a large number of stars at the same time. However, it has a significant limitation: since it is necessary to wait at least three passes in front of the star to confirm the existence of a planet, it is currently only suitable to detect planets with rather short orbital periods (typically from a few days to a few months). We would indeed have to wait more than 30 years to detect a planet similar to Jupiter which needs 11 years to make the full tour).
- To overcome this obstacle, a team of astronomers led by researcher Helen Giles, from the Astronomy Department at the Faculty of Science of UNIGE and a member of the NCCR PlanetS ( National Centre of Competence in Research PlanetS), has developed an original method. By analyzing data from the space telescope K2, one star showed a significant long-duration temporary decrease of luminosity, the signature of a possible transit, in other words, the passage of a planet in front of its star. "We had to analyze hundreds of light curves" explains the astronomer, to find one where such a transit was unequivocal.
- Helen Giles consulted recent data from the Gaia mission to determine the diameter of the star referenced as EPIC248847494 and its distance, 1500 light-years away from the planet Earth. With that knowledge and the fact that the transit lasted 53 hours, she found that the planet is located at 4.5 times the distance from the Sun to the Earth, and that consequently it takes about 10 years to orbit once. The key question left to answer was whether it was a planet and not a star. The Euler telescope of the UNIGE in Chile would provide the answer. By measuring the radial velocity of the star, which makes it possible to deduce the mass of the planet, she was able to show that the mass of the object is less than 13 times that of Jupiter — well below the minimum mass of a star (at least 80 times the mass of Jupiter).
Figure 15: Data from the light curve of the EPIC248847494 star. The transit is clearly visible, on the upper right part of the image (image credit: UNIGE)
- "This technique could be used to hunt habitable, Earth-like planets around stars like the Sun" enthuses Helen Giles, "we have already found Earths around red dwarf stars whose stellar radiation may have consequences on life which are not exactly known". With her method it will no longer be necessary to wait many years to know whether the detected single transit is due to the presence of a planet. "In the future, we could even see if the planet has one or more moons, like our Jupiter," she concludes.
• July 6, 2018: Earlier this week, NASA's Kepler team received an indication that the spacecraft fuel tank is running very low. NASA has placed the spacecraft in a hibernation-like state in preparation to download the science data collected in its latest observation campaign. Once the data has been downloaded, the expectation is to start observations for the next campaign with any remaining fuel. 27)
- Since May 12, Kepler has been on its 18th observation campaign, staring at a patch of sky towards the constellation of Cancer it previously studied in 2015. The data from this second look will provide astronomers with an opportunity to confirm previous exoplanet candidates and discover new ones. Returning the data back to Earth is the highest priority for the remaining fuel.
- To bring the data home, the spacecraft must point its large antenna back to Earth and transmit the data during its allotted Deep Space Network time, which is scheduled in early August. Until then, the spacecraft will remain stable and parked in a no-fuel-use safe mode. On August 2, the team will command the spacecraft to awaken from its no-fuel-use state and maneuver the spacecraft to the correct orientation and downlink the data. If the maneuver and download are successful, the team will begin its 19th observation campaign on August 6 with the remaining fuel.
- NASA will provide an update after the scheduled download. The agency has been monitoring the Kepler spacecraft closely for signs of low fuel, and expects it to run out of fuel in the next few months.
- As engineers preserve the new data stored on the spacecraft, scientists are continuing to mine existing data already on the ground. Among other findings, recently 24 new planet discoveries were made using data from the 10th observation campaign, adding to the spacecraft's growing bounty of 2,650 confirmed planets.
• March 14, 2018: Trailing Earth's orbit at 94 million miles away (~151 million km), the Kepler space telescope has survived many potential knock-outs during its nine years in flight, from mechanical failures to being blasted by cosmic rays. At this rate, the hardy spacecraft may reach its finish line in a manner we will consider a wonderful success. With nowhere a gas station to be found in deep space, the spacecraft is going to run out of fuel. We expect to reach that moment within several months. 28)
- In 2013, Kepler's primary mission ended when a second reaction wheel broke, rendering it unable to hold its gaze steady at the original field of view. The spacecraft was given a new lease on life by using the pressure of sunlight to maintain its pointing, like a kayak steering into the current. Reborn as "K2," this extended mission requires the spacecraft to shift its field of view to new portions of the sky roughly every three months in what we refer to as a "campaign." Initially, the Kepler team estimated that the K2 mission could conduct 10 campaigns with the remaining fuel. It turns out we were overly conservative. The mission has already completed 16 campaigns, and this month entered its 17th.
- Our current estimates are that Kepler's tank will run dry within several months – but we've been surprised by its performance before! So, while we anticipate flight operations ending soon, we are prepared to continue as long as the fuel allows.
- The Kepler team is planning to collect as much science data as possible in its remaining time and beam it back to Earth before the loss of the fuel-powered thrusters means that we can't aim the spacecraft for data transfer. We even have plans to take some final calibration data with the last bit of fuel, if the opportunity presents itself.
- Without a gas gauge, we have been monitoring the spacecraft for warning signs of low fuel— such as a drop in the fuel tank's pressure and changes in the performance of the thrusters. But in the end, we only have an estimate – not precise knowledge. Taking these measurements helps us decide how long we can comfortably keep collecting scientific data.
- It's like trying to decide when to gas up your car. Do you stop now? Or try to make it to the next station? In our case, there is no next station, so we want to stop collecting data while we're still comfortable that we can aim the spacecraft to bring it back to Earth. We will continue to provide updates on the science and the spacecraft, which has yet to show warning signs.
- Many NASA missions must set a course for a clear-cut ending and reserve enough fuel for one last maneuver. For example, Earth-orbiting spacecraft must avoid collisions with other satellites or an uncontrolled fall to the ground, while planetary missions like Cassini have to reserve fuel to avoid contamination of a potentially life-bearing environment. In Cassini's case, NASA sent the spacecraft into Saturn rather than risk it falling into one of the planet's moons. Deep space missions like Kepler are nowhere near Earth or sensitive environments, which means we can afford to squeeze every last drop of data from the spacecraft — and ultimately that means bringing home even more data for science. Who knows what surprises about our universe will be in that final downlink to Earth?
- While Kepler continues to bring us exciting data as it draws close the finish line, the Transiting Exoplanet Survey Satellite (TESS) will be launching on April 16 from Cape Canaveral, Florida. TESS will search nearly the entire sky for planets outside our solar system, focusing on the brightest stars less than 300 light-years away, and adding to Kepler's treasure trove of planet discoveries.
• March 12, 2018: Scientists report the existence of 15 new planets — including one 'super-Earth' that could harbor liquid water — orbiting small, cool stars near our solar system. These stars, known as red dwarfs, are of enormous interest for studies of planetary formation and evolution. 29)
- An international research team led by Teruyuki Hirano of TITech (Tokyo Institute of Technology)'s Department of Earth and Planetary Sciences has validated 15 exoplanets orbiting red dwarf systems.
- One of the brightest red dwarfs, K2-155 that is around 200 light years away from Earth, has three transiting super-Earths, which are slightly bigger than our own planet. Of those three super-Earths, the outermost planet, K2-155d, with a radius 1.6 times that of Earth, could be within the host star's habitable zone.
- The findings, published in the form of two papers in The Astronomical Journal, are based on data from NASA Kepler spacecraft's second mission, K2, and follow-up observations using ground-based telescopes, including the Subaru Telescope in Hawaii and the Nordic Optical Telescope (NOT) in Spain. 30) 31)
- A more precise estimate of the radius and temperature of the K2-155 star would be needed to conclude definitively whether K2-155d is habitable. Achieving such precision would require further studies, for example, using interferometric techniques.
- A key outcome from the current studies was that planets orbiting red dwarfs may have remarkably similar characteristics to planets orbiting solar-type stars.
- "It's important to note that the number of planets around red dwarfs is much smaller than the number around solar-type stars," says Hirano. "Red dwarf systems, especially coolest red dwarfs, are just beginning to be investigated, so they are very exciting targets for future exoplanet research."
- For example, while the so-called radius gap of planets around solar-type stars has been reported previously, this is the first time that researchers have shown a similar gap in planets around red dwarfs. "This is a unique finding, and many theoretical astronomers are now investigating what causes this gap," says Hirano. He adds that the most likely explanation for the lack of large planets in the proximity of host stars is photoevaporation, which can strip away the envelope of the planetary atmosphere.
- The researchers also investigated the relationship between planet radius and metallicity of the host star. "Large planets are only discovered around metal-rich stars," Hirano says, "and what we found was consistent with our predictions. The few planets with a radius about three times that of Earth were found orbiting the most metal-rich red dwarfs."
- The studies were conducted as part of the KESPRINT collaboration, a group formed by the merger of KEST (Kepler Exoplanet Science Team) and ESPRINT (Equipo de Seguimiento de Planetas Rocosos Intepretando sus Transitos) in 2016.
Figure 16: The researchers found that K2-155d could potentially have liquid water on its surface based on three-dimensional global climate simulations. Hirano expresses both excitement and restraint, as he says: "In our simulations, the atmosphere and the composition of the planet were assumed to be Earth-like, and there's no guarantee that this is the case."(image credit: TITech and research team)
• March 7, 2018: Capturing images of our home planet from the perspective of faraway spacecraft has become a tradition at NASA, ever since Voyager, 28 years ago, displayed our "pale blue dot" in the vastness of space. — But the view of Earth from NASA's Kepler Space Telescope is quite something else (Figure 17). 32)
- At 94 million miles away, Kepler's interpretation of Earth as a bright flashlight in a dark sea of stars demonstrates the capabilities of its highly sensitive photometer, which is designed to pick up the faint dips in brightness of planets crossing distant stars. Some stars in this image are hundreds of light years away.
- The scientific community celebrated Earth's transit across Kepler's field of view by using #WaveAtKepler on social media. As Kepler only takes pictures in black and white, some in the science community have taken the data and used color to highlight details in grayscale images.
- The mission marks its nine-year anniversary in space on March 7. More than 2,500 planets have been found in the Kepler data so far, as well as many other discoveries about stars, supernovae and other astrophysical phenomena. The mission is in its second extended operating phase and is known to have a limited lifetime. Its scientific success in discovering distant planets has paved the way for TESS (Transiting Exoplanet Survey Satellite), which is launching on April 16. TESS will monitor more than 200,000 of the brightest and nearest stars outside our solar system for transiting planets.
Figure 17: This Kepler image of Earth was recently beamed back home. Captured on Dec. 10, 2017 after the spacecraft adjusted its telescope to a new field of view, Earth's reflection as it slipped past was so extraordinarily bright that it created a saber-like saturation bleed across the instrument's sensors, obscuring the neighboring Moon (image credit: NASA)
• February 15, 2018: Based on data from NASA's K2 mission, an international team of scientists has confirmed nearly 100 new exoplanets. This brings the total number of new exoplanets found with the K2 mission up to almost 300. 33)
Figure 18: After detecting the first exoplanets in the 1990s it has become clear that planets around other stars are the rule rather than the exception and there are likely hundreds of billions of exoplanets in the Milky Way alone. The search for these planets is now a large field of astronomy (image credit: ESA/Hubble/ESO/M. Kornmesser)
- "We started out analyzing 275 candidates of which 149 were validated as real exoplanets. In turn 95 of these planets have proved to be new discoveries," said American PhD student Andrew Mayo at the National Space Institute (DTU Space) at the Technical University of Denmark. "This research has been underway since the first K2 data release in 2014." Mayo is the main author of the work being presented in the Astronomical Journal. 34)
- The research was conducted partly as a senior project during his undergraduate studies at Harvard College. It also involved a team of international colleagues from institutions such as NASA, Caltech, UC Berkeley, the University of Copenhagen, and the University of Tokyo. The Kepler spacecraft was launched in 2009 to hunt for exoplanets in a single patch of sky, but in 2013, a mechanical failure crippled the telescope. However, astronomers and engineers devised a way to repurpose and save the space telescope by changing its field of view periodically. This solution paved the way for the follow-up K2 mission, which is still ongoing as the spacecraft searches for exoplanet transits.
- These transits can be found by registering dips in light caused by the shadow of an exoplanet as it crosses in front of its host star. These dips are indications of exoplanets, which must then be examined more closely in order to confirm their nature. Exoplanetary research is a relatively young field. The first planet orbiting a star similar to our own sun was detected in 1995. Today some 3,600 exoplanets have been found, ranging from rocky Earth-sized planets to large gas giants like Jupiter.
- It's difficult work to distinguish which signals are actually coming from exoplanets. Mayo and his colleagues analyzed hundreds of signals of potential exoplanets to determine which signals were created by exoplanets and which were caused by other sources. "We found that some of the signals were caused by multiple star systems or noise from the spacecraft. But we also detected planets that range from sub Earth-sized to the size of Jupiter and larger," said Mayo.
- One of the planets detected was orbiting a very bright star. "We validated a planet on a 10-day orbit around a star called HD 212657, which is now the brightest star found by either the Kepler or K2 missions to host a validated planet. Planets around bright stars are important because astronomers can learn a lot about them from ground-based observatories," said Mayo.
- "Exoplanets are a very exciting field of space science. As more planets are discovered, astronomers will develop a much better picture of the nature of exoplanets which in turn will allow us to place our own solar system into a galactic context".
- The Kepler space telescope has made huge contributions to the field of exoplanets both in its original mission and its successor K2 mission. So far these missions have provided over 5,100 exoplanet candidates that can now be examined more closely.
- With new, upcoming space missions like the James Webb Space Telescope and the Transiting Exoplanet Survey Satellite, astronomers will take exciting new steps toward characterizing and studying exoplanets like the rocky, habitable, Earth-sized planets that might be capable of supporting life.
• December 14, 2017: Our solar system now is tied for most number of planets around a single star, with the recent discovery of an eighth planet circling Kepler-90, a Sun-like star 2,545 light years from Earth. The planet was discovered in data from NASA's Kepler Space Telescope. 35)
- The newly-discovered Kepler-90i - a sizzling hot, rocky planet that orbits its star once every 14.4 days - was found using machine learning from Google. Machine learning is an approach to artificial intelligence in which computers "learn." In this case, computers learned to identify planets by finding in Kepler data instances where the telescope recorded changes in starlight caused by planets beyond our solar system, known as exoplanets.
Figure 19: With the discovery of an eighth planet, the Kepler-90 system is the first to tie with our solar system in number of planets. Artist's concept (image credit: NASA/Ames Research Center/Wendy Stenzel) 36)
- "Just as we expected, there are exciting discoveries lurking in our archived Kepler data, waiting for the right tool or technology to unearth them," said Paul Hertz, director of NASA's Astrophysics Division in Washington. "This finding shows that our data will be a treasure trove available to innovative researchers for years to come."
- The discovery came about after researchers Christopher Shallue and Andrew Vanderburg trained a computer to learn how to identify exoplanets in the light readings recorded by Kepler - the miniscule change in brightness captured when a planet passed in front of, or transited, a star. Inspired by the way neurons connect in the human brain, this artificial "neural network" sifted through Kepler data and found weak transit signals from a previously-missed eighth planet orbiting Kepler-90, in the constellation Draco.
- Machine learning has previously been used in searches of the Kepler database, and this continuing research demonstrates that neural networks are a promising tool in finding some of the weakest signals of distant worlds.
- Other planetary systems probably hold more promise for life than Kepler-90. About 30 percent larger than Earth, Kepler-90i is so close to its star that its average surface temperature is believed to exceed 800 degrees Fahrenheit, on par with Mercury. Its outermost planet, Kepler-90h, orbits at a similar distance to its star as Earth does to the Sun.
- "The Kepler-90 star system is like a mini version of our solar system. You have small planets inside and big planets outside, but everything is scrunched in much closer," said Vanderburg, a NASA Sagan Postdoctoral Fellow and astronomer at the University of Texas at Austin.
- Christopher Shallue, a senior software engineer with Google's research team Google AI, came up with the idea to apply a neural network to Kepler data. He became interested in exoplanet discovery after learning that astronomy, like other branches of science, is rapidly being inundated with data as the technology for data collection from space advances.
- "In my spare time, I started Googling for 'finding exoplanets with large data sets' and found out about the Kepler mission and the huge data set available," said Shallue. "Machine learning really shines in situations where there is so much data that humans can't search it for themselves."
- Kepler's four-year dataset consists of 35,000 possible planetary signals. Automated tests, and sometimes human eyes, are used to verify the most promising signals in the data. However, the weakest signals often are missed using these methods. Shallue and Vanderburg thought there could be more interesting exoplanet discoveries faintly lurking in the data.
- First, they trained the neural network to identify transiting exoplanets using a set of 15,000 previously vetted signals from the Kepler exoplanet catalogue. In the test set, the neural network correctly identified true planets and false positives 96 percent of the time. Then, with the neural network having "learned" to detect the pattern of a transiting exoplanet, the researchers directed their model to search for weaker signals in 670 star systems that already had multiple known planets. Their assumption was that multiple-planet systems would be the best places to look for more exoplanets.
- "We got lots of false positives of planets, but also potentially more real planets," said Vanderburg. "It's like sifting through rocks to find jewels. If you have a finer sieve then you will catch more rocks but you might catch more jewels, as well."
- Kepler-90i wasn't the only jewel this neural network sifted out. In the Kepler-80 system, they found a sixth planet. This one, the Earth-sized Kepler-80g, and four of its neighboring planets form what is called a resonant chain - where planets are locked by their mutual gravity in a rhythmic orbital dance. The result is an extremely stable system, similar to the seven planets in the TRAPPIST-1 system.
- Their research paper reporting these findings has been accepted for publication in The Astronomical Journal. Shallue and Vanderburg plan to apply their neural network to Kepler's full set of more than 150,000 stars.
- Kepler has produced an unprecedented data set for exoplanet hunting. After gazing at one patch of space for four years, the spacecraft now is operating on an extended mission and switches its field of view every 80 days.
- "These results demonstrate the enduring value of Kepler's mission," said Jessie Dotson, Kepler's project scientist at NASA's Ames Research Center in California's Silicon Valley. "New ways of looking at the data - such as this early-stage research to apply machine learning algorithms - promise to continue to yield significant advances in our understanding of planetary systems around other stars. I'm sure there are more firsts in the data waiting for people to find them."
• September 2017: Since it was launched in 2009, NASA's Kepler mission has continued to make important exoplanet discoveries. Even after the failure of two reaction wheels, the space observatory has found new life in the form of its K2 mission. 37) 38)
- With the confirmation of over 3500 planets to date and an additional ~4500 candidates from Kepler, the focus of studying exoplanets has largely shifted from pure discovery to understanding planetary demographics, system architectures, interior structures, and atmospheres. In particular, planets which transit their host stars are valuable for understanding the properties of small planets in detail. Like an eclipsing binary star, combining the transit light curve with radial velocity observations yields a measurement of the mass and radius of a planet relative to its star, which constrain the planet's interior structure. Planetary atmospheres can also be studied if the planet transits. The opacity of a planet's atmosphere depends on its chemical composition and the wavelength of the observation. This causes the apparent size of the planet to change as a function of wavelength. Therefore, by measuring the depth of the transit as a function of wavelength, it is possible to gain insight into the composition and temperature of the planet's atmosphere (this technique is known as transit transmission spectroscopy.
- The most recent discovery was made by an international team of astronomers around Gliese 9827 (GJ 9827), a late K-type dwarf star located about 100 light-years from Earth. Using data provided by the K2 mission, they detected the presence of three Super-Earths. This star system is the closest exoplanet-hosting star discovered by K2 to date, which makes these planets well-suited for follow-up studies.
- The study team relied on data obtained during Campaign 12 of the K2 mission – from December 2016 to March 2017. After consulting this data, the team noted the presence of three super-Earth sized planets orbiting in a very compact configuration. This system, as they note in their study, was independently and simultaneously discovered by another team from Wesleyan University.
- These three planetary objects, designated as GJ 9827 b, c, and d, are located at a distance of about 0.02, 0.04 and 0.06 AU from their host star (respectively). Owing to their sizes and radii, these planets are classified as "Super-Earths", and have radii of 1.6, 1.2, and 2.1 times the radius of Earth. They are also located very close to their host star, completing orbits within 6.2 days.
- Specifically, GJ 9827 b measures 1.64 Earth radii, has a mass of up to 4.25 Earth masses, a 1.2 day orbital period, and a temperature of 1,119 K (846 °C). Meanwhile, GJ 9827 c measures 1.29 Earth radii, has a mass of 2.62 Earth masses, an orbital period of 3.6 days, and a temperature of 774 K (500 °C). Lastly, GJ 9827 d measures 2.08 Earth radii, has a mass of 5.3 Earth masses, a 6.2 day period, and a temperature of 648 K (375 °C).
- In short, all three planets are very hot, with temperatures that are as hot as Venus and Mercury or (in the case of GJ 9827b) is even hotter! Interestingly, these radii and mass estimates place these planets within the transition boundary between terrestrial (i.e. rocky) planets and gas giants. In fact, the team found that GJ 9827 b and c fall in or close to the known gap in radius distribution for planets that are in between these two populations.
- In other words, these planets could be rocky or gaseous, and the team won't know for sure until they can place more accurate constraints on their masses. What's more, none of these planets are likely to be capable of supporting life, certainly not as we know it! So if you were hoping that this latest find would produce an Earth-analog or potentially habitable planet, you're sadly mistaken.
- Nevertheless, the fact that these planets straddle the radius and mass boundary between terrestrial and gaseous planets – and the fact that this system is the closest planetary system to be identified by the K2 mission – makes the system well-situated for studies designed to probe the interior structure and atmosphere of exoplanets.
Figure 20: Top: The full K2 light curve of GJ 9827 from Campaign 12, corrected for systematics. Middle: The corrected K2 lightcurve with best-fit low frequency variability removed. Bottom: Phase folded K2 light curves of GJ 9827 b, c. and d. The observations are plotted in open black circles, and the best fit models are plotted in red (image credit: Gliese 9827 Study Team, Ref. 38)
• June 19, 2017: NASA's Kepler space telescope team has released a mission catalog of planet candidates that introduces 219 new planet candidates, 10 of which are near-Earth size and orbiting in their star's habitable zone, which is the range of distance from a star where liquid water could pool on the surface of a rocky planet. 39)
- This is the most comprehensive and detailed catalog release of candidate exoplanets, which are planets outside our solar system, from Kepler's first four years of data. It's also the final catalog from the spacecraft's view of the patch of sky in the Cygnus constellation.
- With the release of this catalog, derived from data publicly available on the NASA Exoplanet Archive, there are now 4,034 planet candidates identified by Kepler. Of which, 2,335 have been verified as exoplanets. Of roughly 50 near-Earth size habitable zone candidates detected by Kepler, more than 30 have been verified.
- Additionally, results using Kepler data suggest two distinct size groupings of small planets. Both results have significant implications for the search for life. The final Kepler catalog will serve as the foundation for more study to determine the prevalence and demographics of planets in the galaxy, while the discovery of the two distinct planetary populations shows that about half the planets we know of in the galaxy either have no surface, or lie beneath a deep, crushing atmosphere – an environment unlikely to host life.
- "The Kepler data set is unique, as it is the only one containing a population of these near Earth-analogs – planets with roughly the same size and orbit as Earth," said Mario Perez, Kepler program scientist in the Astrophysics Division of NASA's Science Mission Directorate. "Understanding their frequency in the galaxy will help inform the design of future NASA missions to directly image another Earth." - The Kepler space telescope hunts for planets by detecting the minuscule drop in a star's brightness that occurs when a planet crosses in front of it, called a transit.
- This is the eighth release of the Kepler candidate catalog, gathered by reprocessing the entire set of data from Kepler's observations during the first four years of its primary mission. This data will enable scientists to determine what planetary populations – from rocky bodies the size of Earth, to gas giants the size of Jupiter – make up the galaxy's planetary demographics.
- To ensure a lot of planets weren't missed, the team introduced their own simulated planet transit signals into the data set and determined how many were correctly identified as planets. Then, they added data that appear to come from a planet, but were actually false signals, and checked how often the analysis mistook these for planet candidates. This work told them which types of planets were overcounted and which were undercounted by the Kepler team's data processing methods.
- "This carefully-measured catalog is the foundation for directly answering one of astronomy's most compelling questions – how many planets like our Earth are in the galaxy?" said Susan Thompson, Kepler research scientist for the SETI Institute in Mountain View, California, and lead author of the catalog study.
- One research group took advantage of the Kepler data to make precise measurements of thousands of planets, revealing two distinct groups of small planets. The team found a clean division in the sizes of rocky, Earth-size planets and gaseous planets smaller than Neptune. Few planets were found between those groupings.
- Using the W. M. Keck Observatory in Hawaii, the group measured the sizes of 1,300 stars in the Kepler field of view to determine the radii of 2,000 Kepler planets with exquisite precision.
- "We like to think of this study as classifying planets in the same way that biologists identify new species of animals," said Benjamin Fulton, doctoral candidate at the University of Hawaii in Manoa, and lead author of the second study. "Finding two distinct groups of exoplanets is like discovering mammals and lizards make up distinct branches of a family tree."
- It seems that nature commonly makes rocky planets up to about 75 percent bigger than Earth. For reasons scientists don't yet understand, about half of those planets take on a small amount of hydrogen and helium that dramatically swells their size, allowing them to "jump the gap" and join the population closer to Neptune's size.
- The Kepler spacecraft continues to make observations in new patches of sky in its extended mission, searching for planets and studying a variety of interesting astronomical objects, from distant star clusters to objects such as the TRAPPIST-1 system of seven Earth-size planets, closer to home.
Figure 21: NASA's Kepler space telescope team has identified 219 new planet candidates, 10 of which are near-Earth size and in the habitable zone of their star (image credit: NASA/JPL-Caltech)
Figure 22: NASA's Kepler space telescope was the first agency mission capable of detecting Earth-size planets using the transit method, a photometric technique that measures the minuscule dimming of starlight as a planet passes in front of its host star. For the first four years of its primary mission, the space telescope observed a set starfield located in the constellation Cygnus (left). New results released from Kepler data June 19, 2017, have implications for understanding the frequency of different types of planets in our galaxy and the way planets are formed. Since 2014, Kepler has been collecting data on its second mission, observing fields on the plane of the ecliptic of our galaxy (right), image credit: NASA/Wendy Stenzel
• May 29, 2017: After 60 hours of non-stop work, researchers at the University of Bern being part of an international team reached their hoped-for goal: They were the first to measure the orbital period for the outermost planet of the famous TRAPPIST-1 system which made The race to trace TRAPPIST-1hs worldwide. The new result confirms that the seven Earth-size planets around the ultra-cool dwarf are lined up in a chain with resonances linking every member. 40)
- "We knew that we had to be very fast to be the first ones", says Marko Sestovic, PhD student at the University of Bern's CSH (Center for Space and Habitability). "If you are not the first, nobody cares about your effort", adds CSH postdoc Simon Grimm.
- NASA had started the global research race with announcing the public release of data about two months ahead of schedule. Its Kepler Space Telescope had observed the star TRAPPIST-1 from mid-December until early March. Previous observations with telescopes on the ground and in space had revealed that the ultra-cool dwarf is orbited by seven Earth-size planets, a larger number than detected in any other transiting exoplanetary system. Whereas the astrophysicists could determine the orbital periods for the first six planets, the period of the outermost planet remained unconstrained.
- Only four days after the end of the Kepler observing campaign, NASA made the dataset publicly available. The researchers in Bern had already teamed up with colleagues at the University of Washington and other scientists in the US, Belgium, France, UK, Saudi Arabia and Morocco to be able to start work immediately.
- Simon Grimm had developed a tool to automatically download and process the data as soon as they were made available, whereas Marko Sestovic developed a pipeline to perform the data analysis. The same job was independently done by Rodrigo Luger, a PhD student at the University of Washington in Seattle.
- Removing the noise: Normally, NASA calibrates the data of the Kepler Space Telescope in a lengthy procedure before public release, but this time the researchers got only the raw dataset. "Due to technical problems, the Kepler spacecraft is no longer able to maintain its pointing direction accurately, causing it to drift and jitter as its thrusters fight to keep it pointed at the star", explains Sestovic: "This introduces strong instrumental signals and a lot of noise that must be removed. The noise can be several times bigger than the transit signal we are looking for".
- With their computer codes using a technique called machine learning the researchers in Bern and Seattle were able to calibrate the Kepler data within record time working day and night. "Every hour counts when there is a risk of being scooped", says CSH professor Brice-Olivier Demory.
- To determine the orbital period of the outermost planet called TRAPPIST-1h the researchers counted on theory. They had already found that because of gravitational interactions the periods of the inner planets were linked: For instance, if planet b orbits the star eight times, planet c completes five orbits, and planet d three - a phenomena called mean-motion resonance.
- If planet h was also in resonance with its neighboring planets f and g, the period would have one of certain discrete values. Examining previous data, Brice-Olivier Demory as well as two colleagues in Seattle and Chicago were able to rule out all but one of those possible periods: 18.764 Earth days. "Remarkably, we found the planet in the Kepler data exactly where theory predicted", says Demory. So, if planet h orbits TRAPPIST-1 twice, its inner neighbor g completes 3 orbits, and f four.
- In addition to the period of TRAPPIST-1h, the scientists found that the temperature of the planet is 169 K, or -104 degrees Celsius and its radius is a bit smaller than the Earth radius. They also measured that the star itself rotates once every 3.3 days, and inferred from the low activity of TRAPPIST-1 that it is older than previously expected.
- Once NASA had released the data, the researchers in Bern and their colleagues worked uninterrupted during 60 hours, analyzing the data and writing up the findings in a 37-page paper which is now being published in the journal Nature Astronomy. "Within our collaboration of 30 people, more than 450 emails were sent between all of us in just a few days", summarizes Brice-Olivier Demory: "At some point 15 people where simultaneously editing the online manuscript - a nice experience overall!" 41)
Figure 23: The orbits of the seven planets around the star TRAPPIST-1. The grey region is the zone, where liquid water could exist on the surface of the planets. On planet TRAPPIST-1 h liquid water is possible under a thick layer of ice (1 AU is the distance between the Sun and the Earth: ~150 million km), image credit: A. Triaud
• May 22, 2017: Scientists using NASA's Kepler space telescope identified a regular pattern in the orbits of the planets in the TRAPPIST-1 system that confirmed suspected details about the orbit of its outermost and least understood planet, TRAPPIST-1h. 42)
- TRAPPIST-1 is only eight percent the mass of our sun, making it a cooler and less luminous star. It's home to seven Earth-size planets, three of which orbit in their star's habitable zone — the range of distances from a star where liquid water could pool on the surface of a rocky planet. The system is located about 40 light-years away in the constellation of Aquarius. The star is estimated to be between 3 billion and 8 billion years old.
- Scientists announced that the system has seven Earth-sized planets at a NASA press conference on Feb. 22. NASA's Spitzer Space Telescope, the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) in Chile and other ground-based telescopes were used to detect and characterize the planets. But the collaboration only had an estimate for the period of TRAPPIST-1h.
- Astronomers from the University of Washington have used data from the Kepler spacecraft to confirm that TRAPPIST-1h orbits its star every 19 days. At six million miles from its cool dwarf star, TRAPPIST-1h is located beyond the outer edge of the habitable zone, and is likely too cold for life as we know it. The amount of energy (per unit area) planet h receives from its star is comparable to what the dwarf planet Ceres, located in the asteroid belt between Mars and Jupiter, gets from our sun.
- "It's incredibly exciting that we're learning more about this planetary system elsewhere, especially about planet h, which we barely had information on until now," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate at Headquarters in Washington. "This finding is a great example of how the scientific community is unleashing the power of complementary data from our different missions to make such fascinating discoveries."
Figure 24: This artist's concept shows TRAPPIST-1h, one of seven Earth-size planets in the TRAPPIST-1 planetary system. NASA's Kepler spacecraft, operating in its K2 mission, obtained data that allowed scientists to determine that the orbital period of TRAPPIST-1h is 19 days (image credit: NASA/JPL-Caltech)
• March 8, 2017: On Feb. 22, astronomers announced that the ultra-cool dwarf star, TRAPPIST-1, hosts a total of seven Earth-size planets that are likely rocky, a discovery made by NASA's Spitzer Space Telescope in combination with ground-based telescopes. NASA's planet-hunting Kepler space telescope also has been observing this star since December 2016. Today these additional data about TRAPPIST-1 from Kepler are available to the scientific community. 43)
- During the period of Dec. 15, 2016 to March 4, 2017 the Kepler spacecraft, operating as the K2 mission, collected data on the star's minuscule changes in brightness due to transiting planets. These additional observations are expected to allow astronomers to refine the previous measurements of six planets, pin down the orbital period and mass of the seventh and farthest planet, TRAPPIST-1h, and learn more about the magnetic activity of the host star.
- "Scientists and enthusiasts around the world are interested in learning everything they can about these Earth-size worlds," said Geert Barentsen, K2 research scientist at NASA/ARC (Ames Research Center) at Moffett Field, California. "Providing the K2 raw data as quickly as possible was a priority to give investigators an early look so they could best define their follow-up research plans. We're thrilled that this will also allow the public to witness the process of discovery."
- The release of the raw, uncalibrated data collected will aid astronomers in preparing proposals due this month to use telescopes on Earth next winter to further investigate TRAPPIST-1. By late May, the routine processing of the data will be completed and the fully calibrated data will be made available at the public archive.
- The observation period, known as K2 Campaign 12, provides 74 days of monitoring. This is the longest, nearly continuous set of observations of TRAPPIST-1 yet, and provides researchers with an opportunity to further study the gravitational interaction between the seven planets, and search for planets that may remain undiscovered in the system.
- TRAPPIST-1 wasn't always on the radar to study. In fact, the initial coordinates for the patch of sky defined as Campaign 12 were set in Oct. 2015. That was before the planets orbiting TRAPPIST-1 were known to exist, so Kepler would have just missed the region of space that is home to this newfound star system of interest.
- But in May 2016, when the discovery of three of TRAPPIST-1's planets was first announced, the teams at NASA and Ball Aerospace & Technologies Corp. quickly reworked the calculations and rewrote and tested the commands that would be programmed into the spacecraft's operating system to make a slight pointing adjustment for Campaign 12. By Oct. 2016, Kepler was ready and waiting to begin the study of our intriguing neighbor in the constellation Aquarius.
- "We were lucky that the K2 mission was able to observe TRAPPIST-1. The observing field for Campaign 12 was set when the discovery of the first planets orbiting TRAPPIST-1 was announced, and the science community had already submitted proposals for specific targets of interest in that field," said Michael Haas, science office director for the Kepler and K2 missions at Ames. "The unexpected opportunity to further study the TRAPPIST-1 system was quickly recognized and the agility of the K2 team and science community prevailed once again."
- The added refinements to the previous measurements of the known planets and any additional planets that may be discovered in the K2 data will help astronomers plan for follow-up studies of the neighboring TRAPPIST-1 worlds using NASA's upcoming James Webb Space Telescope.
- During Campaign 12, a cosmic ray event reset the spacecraft's onboard software, causing a five-day break in science data collection. The benign event is the fourth occurrence of cosmic ray susceptibility since launch in March 2009. The spacecraft remains healthy and is operating nominally.
Figure 25: This illustration shows the seven TRAPPIST-1 planets as they might look as viewed from Earth using a fictional, incredibly powerful telescope. (image credit: NASA/JPL-Caltech)
Legend to Figure 25: The sizes and relative positions are correctly to scale: This is such a tiny planetary system that its sun, TRAPPIST-1, is not much bigger than our planet Jupiter, and all the planets are very close to the size of Earth. Their orbits all fall well within what, in our solar system, would be the orbital distance of our innermost planet, Mercury. With such small orbits, the TRAPPIST-1 planets complete a "year" in a matter of a few Earth days: 1.5 for the innermost planet, TRAPPIST-1b, and 20 for the outermost, TRAPPIST-1h.
• November 16, 2016: Stars are not perfect spheres. While they rotate, they become flat due to the centrifugal force. A team of researchers around Laurent Gizon from the MPS (Max Planck Institute for Solar System Research) and the University of Göttingen has now succeeded in measuring the oblateness of a slowly rotating star with unprecedented precision. The researchers have determined stellar oblateness using asteroseismology – the study of the oscillations of stars. The technique is applied to a star 5000 light years away from Earth and revealed that the difference between the equatorial and polar radii of the star is only 3 km – a number that is astonishing small compared to the star's mean radius of 1.5 million km; which means that the gas sphere is astonishingly round. 44) 45)
- All stars rotate and are therefore flattened by the centrifugal force. The faster the rotation, the more oblate the star becomes. Our Sun rotates with a period of 27 days and has a radius at the equator that is 10 km larger than at the poles; for the Earth this difference is 21 km. Laurent Gizon and his colleagues selected a slowly rotating star named Kepler 11145123. This hot and luminous star is more than twice the size of the Sun and rotates three times more slowly than the Sun.
- Gizon and his colleagues selected this star to study because it supports purely sinusoidal oscillations. The periodic expansions and contractions of the star can be detected in the fluctuations in brightness of the star. NASA's Kepler mission observed the star's oscillations continuously for more than four years. Different modes of oscillation are sensitive to different stellar latitudes. For their study, the authors compare the frequencies of the modes of oscillation that are more sensitive to the low-latitude regions and the frequencies of the modes that are more sensitive to higher latitudes. This comparison shows that the difference in radius between the equator and the poles is only 3 km with a precision of 1 km. "This makes Kepler 11145123 the roundest natural object ever measured, even more round than the Sun" explains Gizon.
- Surprisingly, the star is even less oblate than implied by its rotation rate. The authors propose that the presence of a magnetic field at low latitudes could make the star look more spherical to the stellar oscillations. Just like helioseismology can be used to study the Sun's magnetic field, asteroseismology can be used to study magnetism on distant stars. Stellar magnetic fields, especially weak magnetic fields, are notoriously difficult to directly observe on distant stars.
- Kepler 11145123 is not the only star with suitable oscillations and precise brightness measurements. "We intend to apply this method to other stars observed by Kepler and the upcoming space missions TESS (Transiting Exoplanet Survey Satellite) of NASA and PLATO (Planetary Transits and Oscillations of stars) of ESA. It will be particularly interesting to see how faster rotation and a stronger magnetic field can change a star's shape," Gizon adds, "An important theoretical field in astrophysics has now become observational."
- The star KIC 11145123 belongs to the class of hybrid pulsators. It oscillates both in a high-frequency band (15 to 25 day-1) and in a low frequency band (below 5 day-1). The observed modes of oscillation are acoustic (p), gravity (g), and mixed (p and g) modes. Modes with dominant p-mode character are seen in the high-frequency band. KIC11145123 has a Kepler magnitude Kp = 13, and is a late A star. From the KIC (Kepler Input Catalog) revised photometry, its effective temperature is 8050 ± 200 K and its surface gravity is log g = 4.0±0.2 (cgs units), showing it to be a main sequence A star. 46)
Figure 26: The star Kepler 11145123 is the roundest natural object ever measured in the universe. Stellar oscillations imply a difference in radius between the equator and the poles of only 3 km. This star is significantly more round than the Sun (image credit: MPS Göttingen, Mark A. Garlick)
• August 12, 2016: Like cosmic ballet dancers, the stars of the Pleiades cluster are spinning. But these celestial dancers are all twirling at different speeds. Astronomers have long wondered what determines the rotation rates of these stars. By watching these stellar dancers, NASA's Kepler space telescope during its K2 mission has helped amass the most complete catalog of rotation periods for stars in a cluster. This information can help astronomers gain insight into where and how planets form around these stars, and how such stars evolve. 47)
- "We hope that by comparing our results to other star clusters, we will learn more about the relationship between a star's mass, its age, and even the history of its solar system," said Luisa Rebull, a research scientist at the Infrared Processing and Analysis Center at Caltech in Pasadena, California. She is the lead author of two new papers and a co-author on a third paper about these findings, all being published in the Astronomical Journal. 48) 49)
- The Pleiades star cluster is one of the closest and most easily seen star clusters, residing just 445 light-years away from Earth, on average. At about 125 million years old, these stars — known individually as Pleiads — have reached stellar "young adulthood." In this stage of their lives, the stars are likely spinning the fastest they ever will.
- As a typical star moves further along into adulthood, it loses some zip due to the copious emission of charged particles known as a stellar wind (in our solar system, we call this the solar wind). The charged particles are carried along the star's magnetic fields, which overall exerts a braking effect on the rotation rate of the star.
- Rebull and colleagues sought to delve deeper into these dynamics of stellar spin with Kepler. Given its field of view on the sky, Kepler observed approximately 1,000 stellar members of the Pleiades over the course of 72 days. The telescope measured the rotation rates of more than 750 stars in the Pleiades, including about 500 of the lowest-mass, tiniest, and dimmest cluster members, whose rotations could not previously be detected from ground-based instruments.
- Kepler measurements of starlight infer the spin rate of a star by picking up small changes in its brightness. These changes result from "starspots" which, like the more-familiar sunspots on our sun, form when magnetic field concentrations prevent the normal release of energy at a star's surface. The affected regions become cooler than their surroundings and appear dark in comparison.
- As stars rotate, their starspots come in and out of Kepler's view, offering a way to determine spin rate. Unlike the tiny, sunspot blemishes on our middle-aged sun, starspots can be gargantuan in stars as young as those in the Pleiades because stellar youth is associated with greater turbulence and magnetic activity. These starspots trigger larger brightness decreases, and make spin rate measurements easier to obtain.
- During its observations of the Pleiades, a clear pattern emerged in the data: More massive stars tended to rotate slowly, while less massive stars tended to rotate rapidly. The big-and-slow stars' periods ranged from one to as many as 11 Earth-days. Many low-mass stars, however, took less than a day to complete a pirouette. (For comparison, our sedate sun revolves fully just once every 26 days.) The population of slow-rotating stars includes some ranging from a bit larger, hotter and more massive than our sun, down to other stars that are somewhat smaller, cooler and less massive. On the far end, the fast-rotating, fleet-footed, lowest-mass stars possess as little as a tenth of our sun's mass. "In the 'ballet' of the Pleiades, we see that slow rotators tend to be more massive, whereas the fastest rotators tend to be very light stars," said Rebull.
- The main source of these differing spin rates is the internal structure of the stars, Rebull and colleagues suggest. Larger stars have a huge core enveloped in a thin layer of stellar material undergoing a process called convection, familiar to us from the circular motion of boiling water. Small stars, on the other hand, consist almost entirely of convective, roiling regions. As stars mature, the braking mechanism from magnetic fields more easily slows the spin rate of the thin, outermost layer of big stars than the comparatively thick, turbulent bulk of small stars.
- Thanks to the Pleiades' proximity, researchers think it should be possible to untangle the complex relationships between stars' spin rates and other stellar properties. Those stellar properties, in turn, can influence the climates and habitability of a star's hosted exoplanets. For instance, as spinning slows, so too does starspot generation, and the solar storms associated with starspots. Fewer solar storms means less intense, harmful radiation blasting into space and irradiating nearby planets and their potentially emerging biospheres.
- "The Pleiades star cluster provides an anchor for theoretical models of stellar rotation going both directions, younger and older," said Rebull. "We still have a lot we want to learn about how, when and why stars slow their spin rates and hang up their 'dance shoes,' so to speak." Rebull and colleagues are now analyzing K2 mission data from an older star cluster, Praesepe, popularly known as the Beehive Cluster, to further explore this phenomenon in stellar structure and evolution.
- "We're really excited that K2 data of star clusters, such as the Pleiades, have provided astronomers with a bounty of new information and helped advance our knowledge of how stars rotate throughout their lives," said Steve Howell, project scientist for the K2 mission at NASA's Ames Research Center in Moffett Field, California.
Figure 27: This image shows the Pleiades cluster of stars as seen through the eyes of NASA's WISE (Wide-field Infrared Survey Explorer) mission (image credit: NASA/JPL-Caltech/UCLA)
• August 12, 2016: The Kepler spacecraft's photometer—the onboard camera—was commanded to return to science after being found to be off during a routine contact on Thursday, July 28. Yesterday, the team confirmed that the photometer responded as expected and began collecting data again on Tuesday, Aug. 2. Yesterday's scheduled contact with the spacecraft was made using the 70 m dish of NASA's DSN (Deep Space Network) at Goldstone, CA. The large dish provided the necessary communications link with the spacecraft to confirm Kepler was back doing science. 50)
- Our investigation of the cause centered on the focal plane detector modules. The signature of the recorded faults surrounding the anomaly was reminiscent of an event in January 2014, when one of the science detector modules (Module 7) failed. In that case, we concluded that the most likely cause was a random part failure that resulted in a high electric current in the circuitry, which blew the protection fuse, disabling the detector but preventing the problem from propagating to other detectors. As part of the fault protection response, the photometer was powered off. Thus, it seemed likely that this most recent anomaly might be the result of another random detector failure—something to be expected as we continue to extend the spacecraft's mission.
- Analysis of the temperature data from the focal plane seems to bear out this hypothesis, and points suspicion to science detector Module 4, as the likely culprit. When we turned the photometer back on, all the other modules warmed up at a consistent rate, while Module 4 reacted more slowly, and never reached full operating temperature before the DSN contact ended.
- As part of our standard contact procedures yesterday, we brought down a few pixels from the focal plane to verify the precise pointing of the spacecraft, and some of these pixels were from the suspect Module 4. The data from these pixels failed to register its assigned target star, while pixels from other modules produced the expected signals from their assigned target stars. Therefore we are relatively certain that this detector has indeed failed.
- For the K2 mission's current Campaign 10, the targets that were assigned to Module 4 will yield no further science results. For the campaigns going forward, we will select targets which fall on the remaining operational detectors, which will have little to no impact to the upcoming science.
- Such hardware failures were foreseen in the initial mission planning, and the system design is robust and compartmentalized to minimize the impacts. After more than seven years in the harsh conditions of space, 85 % of Kepler's original detectors are still operating. Module 4 will be the third of the 21 science modules to have failed in-flight.
• July 18, 2016: An international team of astronomers has discovered and confirmed a treasure trove of new worlds using NASA's Kepler spacecraft on its K2 mission. Among the findings tallying 197 initial planet candidates, scientists have confirmed 104 planets outside our solar system. Among the confirmed is a planetary system comprising four promising planets that could be rocky. 51)
- The planets, all between 20 and 50 percent larger than Earth by diameter, are orbiting the M dwarf star K2-72, found 181 light years away in the direction of the Aquarius constellation. The host star is less than half the size of the sun and less bright. The planets' orbital periods range from five and a half to 24 days, and two of them may experience irradiation levels from their star comparable to those on Earth. Despite their tight orbits — closer than Mercury's orbit around the sun — the possibility that life could arise on a planet around such a star cannot be ruled out, according to lead author Crossfield, a Sagan Fellow at the University of Arizona's Lunar and Planetary Laboratory.
- The researchers achieved this extraordinary "roundup" of exoplanets by combining data with follow-up observations by earth-based telescopes including the North Gemini telescope and the W. M. Keck Observatory in Hawaii, the Automated Planet Finder of the University of California Observatories, and the Large Binocular Telescope operated by the University of Arizona. The discoveries are published online in the Astrophysical Journal Supplement Series. 52)
- Both Kepler and its K2 mission discover new planets by measuring the subtle dip in a star's brightness caused by a planet passing in front of its star. In its initial mission, Kepler surveyed just one patch of sky in the northern hemisphere, determining the frequency of planets whose size and temperature might be similar to Earth orbiting stars similar to our sun. In the spacecraft's extended mission in 2013, it lost its ability to precisely stare at its original target area, but a brilliant fix created a second life for the telescope that is proving scientifically fruitful.
- After the fix, Kepler started its K2 mission, which has provided an ecliptic field of view with greater opportunities for Earth-based observatories in both the northern and southern hemispheres. Additionally, the K2 mission is entirely community-driven with all targets proposed by the scientific community.
- Because it covers more of the sky, the K2 mission is capable of observing a larger fraction of cooler, smaller, red-dwarf type stars, and because such stars are much more common in the Milky Way than sun-like stars, nearby stars will predominantly be red dwarfs.
- "An analogy would be to say that Kepler performed a demographic study, while the K2 mission focuses on the bright and nearby stars with different types of planets," said Ian Crossfield. "The K2 mission allows us to increase the number of small, red stars by a factor of 20, significantly increasing the number of astronomical 'movie stars' that make the best systems for further study."
- To validate candidate planets identified by K2, the researchers obtained high-resolution images of the planet-hosting stars as well as high-resolution optical spectroscopy. By dispersing the starlight as through a prism, the spectrographs allowed the researchers to infer the physical properties of a star — such as mass, radius and temperature — from which the properties of any planets orbiting it can be inferred.
Figure 28: Artist concept. A crop of more than 100 planets, discovered by NASA's Kepler Space Telescope, includes four in Earth's size-range orbiting a single dwarf star. Two of these planets are too hot to support life as we know it, but two are in the star's "habitable" zone, where liquid water could exist on the surface. These small, rocky worlds are far closer to their star than Mercury is to our sun. But because the star is smaller and cooler than ours, its habitable zone is much closer. One of the two planets in the habitable zone, K2-72c, has a "year" about 15 Earth-days long—the time it takes to complete one orbit. This closer planet is likely about 10% warmer than Earth. On the second, K2-72e, a year lasts 24 Earth days, this slightly more distant planet would be about 6% colder than Earth (image credit: NASA/JPL)
• May 11, 2016: Dwarf planets tend to be a mysterious bunch. With the exception of Ceres, which resides in the main asteroid belt between Mars and Jupiter, all members of this class of minor planets in our solar system lurk in the depths beyond Neptune. They are far from Earth – small and cold – which makes them difficult to observe, even with large telescopes. So it's little wonder astronomers only discovered most of them in the past decade or so. 53)
- Pluto is a prime example of this elusiveness. Before NASA's New Horizons spacecraft visited it in 2015, the largest of the dwarf planets had appeared as little more than a fuzzy blob, even to the keen-eyed Hubble Space Telescope. Given the inherent challenges in trying to observe these far-flung worlds, astronomers often need to combine data from a variety of sources in order to tease out basic details about their properties.
- Recently, a group of astronomers did just that by combining data from two space observatories to reveal something surprising: a dwarf planet named 2007 OR10 is significantly larger than previously thought. The results peg 2007 OR10 as the largest unnamed world in our solar system and the third largest of the current roster of about half a dozen dwarf planets. The study also found that the object is quite dark and rotating more slowly than almost any other body orbiting our sun, taking close to 45 hours to complete its daily spin.
- For their research, the scientists used NASA's repurposed planet-hunting Kepler space telescope — its mission now known as K2 — along with the archival data from the infrared Herschel Space Observatory. Herschel was a mission of ESA ( European Space Agency) with NASA participation. The research paper reporting these results is published in The Astronomical Journal. 54)
- "K2 has made yet another important contribution in revising the size estimate of 2007 OR10. But what's really powerful is how combining K2 and Herschel data yields such a wealth of information about the object's physical properties," said Geert Barentsen, Kepler/K2 research scientist at NASA/ARC in California's Silicon Valley.
- The revised measurement of the planet's diameter, 1,535 km, is about 100 km greater than the next largest dwarf planet, Makemake, or about one-third smaller than Pluto. Another dwarf planet, named Haumea, has an oblong shape that is wider on its long axis than 2007 OR10, but its overall volume is smaller.
- Like its predecessor mission, K2 searches for the change in brightness of distant objects. The tiny, telltale dip in the brightness of a star can be the signature of a planet passing or transiting in front. But, closer to home, K2 also looks out into our solar system to observe small bodies such as comets, asteroids, moons and dwarf planets. Because of its exquisite sensitivity to small changes in brightness, the Kepler spacecraft is an excellent instrument for observing the brightness of distant solar system objects and how that changes as they rotate.
- Figuring out the size of small, faint objects far from Earth is tricky business. Since they appear as mere points of light, it can be a challenge to determine whether the light they emit represents a smaller, brighter object, or a larger, darker one. This is what makes it so difficult to observe 2007 OR10 — although its elliptical orbit brings it nearly as close to the sun as Neptune, it is currently twice as far from the sun as Pluto.
- Enter the dynamic duo of Kepler and Herschel: Previous estimates based on Herschel data alone suggested a diameter of roughly 1,280 km for 2007 OR10. However, without a handle on the object's rotation period, those studies were limited in their ability to estimate its overall brightness, and hence its size. The discovery of the very slow rotation by K2 was essential for the team to construct more detailed models that revealed the peculiarities of this dwarf planet. The rotation measurements even included hints of variations in brightness across its surface.
- Together, the two space telescopes allowed the team to measure the fraction of sunlight reflected by 2007 OR10 (using Kepler) and the fraction absorbed and later radiated back as heat (using Herschel). Putting these two data sets together provided an unambiguous estimation of the dwarf planet's size and how reflective it is.
- According to the new measurements, the diameter of 2007 OR10 is some 250 km larger than previously thought. The larger size also implies higher gravity and a very dark surface — the latter because the same amount of light is being reflected by a larger body. This dark nature is different from most dwarf planets, which are much brighter. Previous ground-based observations found 2007 OR10 has a characteristic red color, and other researchers have suggested this might be due to methane ices on its surface.
- "Our revised larger size for 2007 OR10 makes it increasingly likely the planet is covered in volatile ices of methane, carbon monoxide and nitrogen, which would be easily lost to space by a smaller object," said András Pál at Konkoly Observatory in Budapest, Hungary, who led the research. "It's thrilling to tease out details like this about a distant, new world — especially since it has such an exceptionally dark and reddish surface for its size."
- As for when 2007 OR10 will finally get a name, that honor belongs to the object's discoverers. Astronomers Megan E. Schwamb, Michael E. Brown and David L. Rabinowitz spotted it in 2007 as part of a survey to search for distant solar system bodies using the Samuel Oschin Telescope at Palomar Observatory near San Diego, California.
- "The names of Pluto-sized bodies each tell a story about the characteristics of their respective objects. In the past, we haven't known enough about 2007 OR10 to give it a name that would do it justice," said Schwamb. "I think we're coming to a point where we can give 2007 OR10 its rightful name."
Figure 29: New K2 results peg 2007 OR10 as the largest unnamed body in our solar system and the third largest of the current roster of about half a dozen dwarf planets. The revised measurement of 2007 OR10's diameter, 1,535 km, is about 100 km greater than the next largest dwarf planet, Makemake, or about one-third smaller than Pluto. Another dwarf planet, named Haumea, has an oblong shape that is wider on its long axis than 2007 OR10, but its overall volume is smaller (image credit: Konkoly Observatory/András Pál, Hungarian Astronomical Association/Iván Éder, NASA/JHUAPL/SwRI)
• May 10, 2016: NASA's Kepler mission has verified 1,284 new planets – the single largest finding of planets to date. 550 of the 1,284 new Kepler planets are small possibly rocky; 9 of those reside in habitable zone. 55)
- Analysis was performed on the Kepler space telescope's July 2015 planet candidate catalog, which identified 4,302 potential planets. For 1,284 of the candidates, the probability of being a planet is greater than 99 percent – the minimum required to earn the status of "planet." An additional 1,327 candidates are more likely than not to be actual planets, but they do not meet the 99 percent threshold and will require additional study. The remaining 707 are more likely to be some other astrophysical phenomena. This analysis also validated 984 candidates previously verified by other techniques.
- "Before the Kepler space telescope launched, we did not know whether exoplanets were rare or common in the galaxy. Thanks to Kepler and the research community, we now know there could be more planets than stars," said Paul Hertz, Astrophysics Division director at NASA Headquarters. "This knowledge informs the future missions that are needed to take us ever-closer to finding out whether we are alone in the universe."
Figure 30: The histogram shows the number of planet discoveries by year for more than the past two decades of the exoplanet search. The blue bar shows previous non-Kepler planet discoveries, the light blue bar shows previous Kepler planet discoveries, the orange bar displays the 1,284 new validated planets (image credit: NASA Ames / W. Stenzel; Princeton University / T. Morton) 56)
• April 7, 2016: Astronomers have made great strides in discovering planets outside of our solar system, termed "exoplanets." In fact, over the past 20 years more than 5,000 exoplanets have been detected beyond the eight planets that call our solar system home. 57)
- The majority of these exoplanets have been found snuggled up to their host star completing an orbit (or year) in hours, days or weeks, while some have been found orbiting as far as Earth is to the sun, taking one-Earth-year to circle. But, what about those worlds that orbit much farther out, such as Jupiter and Saturn, or, in some cases, free-floating exoplanets that are on their own and have no star to call home? In fact, some studies suggest that there may be more free-floating exoplanets than stars in our galaxy.
- This week, NASA's K2 mission, the repurposed mission of the Kepler space telescope, and other ground-based observatories have teamed up to kick-off a global experiment in exoplanet observation. Their mission: survey millions of stars toward the center of our Milky Way galaxy in search of distant stars' planetary outposts and exoplanets wandering between the stars.
- While today's planet-hunting techniques have favored finding exoplanets near their sun, the outer regions of a planetary system have gone largely unexplored. In the exoplanet detection toolkit, scientists have a technique well suited to search these farthest outreaches and the space in between the stars. This technique is called gravitational microlensing.
Figure 31: As an exoplanet passes in front of a more distant star, its gravity causes the trajectory of the starlight to bend, and in some cases results in a brief brightening of the background star as seen by a telescope. The artistic concept illustrates this effect. This phenomenon of gravitational microlensing enables scientists to search for exoplanets that are too distant and dark to detect any other way (image credit: NASA Ames/JPL-Caltech/T. Pyle)
Gravitational Microlensing: For this experiment, astronomers rely on the effect of a familiar fundamental force of nature to help detect the presence of these far out worlds— gravity. The gravity of massive objects such as stars and planets produces a noticeable effect on other nearby objects.
But gravity also influences light, deflecting or warping, the direction of light that passes close to massive objects. This bending effect can make gravity act as a lens, concentrating light from a distant object, just as a magnifying glass can focus the light from the sun. Scientists can take advantage of the warping effect by measuring the light of distant stars, looking for a brightening that might be caused by a massive object, such as a planet, that passes between a telescope and a distant background star. Such a detection could reveal an otherwise hidden exoplanet.
"The chance for the K2 mission to use gravity to help us explore exoplanets is one of the most fantastic astronomical experiments of the decade," said Steve Howell, project scientist for NASA's Kepler and K2 missions at NASA's Ames Research Center in California's Silicon Valley. "I am happy to be a part of this K2 campaign and look forward to the many discoveries that will be made."
This phenomenon of gravitational microlensing – "micro" because the angle by which the light is deflected is small – is the effect for which scientists will be looking during the next three months. As an exoplanet passes in front of a more distant star, its gravity causes the trajectory of the starlight to bend, and in some cases results in a brief brightening of the background star as seen by the observatory. The lensing events caused by a free-floating exoplanet last on the order of a day or two, making the continuous gaze of the Kepler spacecraft an invaluable asset for this technique.
The ground-based observatories will record simultaneous measurements of these brief events. From their different vantage points, space and Earth, the measurements can determine the location of the lensing foreground object through a technique called parallax.
Figure 32: The artistic concept illustrates the relative locations of the search areas for NASA's K2 and Kepler missions (image credit: NASA Ames/W. Stenzel and JPL-Caltech/R. Hurt)
Legend to Figure 32: In a global experiment in exoplanet observation, the K2 mission and Earth-based observatories on six continents will survey millions of stars toward the center of our Milky Way galaxy. Using a technique called gravitational microlensing, scientists will hunt for exoplanets that orbit far from their host star, such as Jupiter is to our sun, and for free-floating exoplanets that wander between the stars. The method allow exoplanets to be found that are up to 10 times more distant than those found by the original Kepler mission, which used the transit technique.
• July 1, 2015: Kepler's Borucki Retires after Five Decades at NASA. After a career spanning 53 years and championing a mission deemed impossible for decades, William Borucki, the principal investigator of NASA's planet-hunting Kepler mission, will retire from the agency on July 3. 58)
- Borucki's civil service at NASA's Ames Research Center in Moffett Field, California, culminated with the development and launch of NASA's first mission to detect Earth-size planets around other stars in the habitable zone — the range of distances from the host star where liquid water might exist on the surface of an orbiting planet. Since its launch in March 2009, Kepler has made scientists and enthusiasts alike re-imagine the possibilities for life in the galaxy.
- Acknowledging Kepler's achievements, Borucki was recently awarded the esteemed Shaw Prize in Astronomy 2015 for conceiving and leading the Kepler mission, which greatly advanced knowledge of both extrasolar planetary systems and stellar interiors. This $1 million award capstone is on top of recognition from U.S. President Obama and many prestigious national space and science foundations.
- In 1983, Borucki began working on what would be approved 17 years later as Kepler with its selection as the 10th Discovery class mission.
• May 15, 2015: It was nearly a year ago that the Kepler spacecraft made a comeback after a technical setback: the loss of its ultra precise pointing capability. Revived as the K2 mission, now in its fifth observing campaign, the spacecraft continues to operate beautifully. - K2 began Campaign 5 on April 26. The observation targets include more than 25,000 stars, which can be searched for exoplanets and examined for a variety of astrophysical phenomena. The field of study also includes M67, an open cluster home to thousands of stars younger than our sun and 2002 YH140, a dwarf planet orbiting beyond Neptune. The field is in the direction of the constellation Cancer. 59)
- Data collected for Campaigns 0, 1 and 2 have been made available to the public through the MAST (Mikulski Archive for Space Telescopes). Campaign 3 data are scheduled for delivery to MAST in June 2015 and Campaign 4 data will be processed with a delivery to MAST planned for August 2015.
- During Campaign 3, observations of Neptune and two of its moons were conducted. Lead researcher, Jason Rowe, SETI Institute Kepler scientist at NASA's Ames Research Center at Moffett Field, California, stitched together more than 100,000 images to generate a movie of the Neptunian system. The movie illustrates 70 days of uninterrupted observation making this one of the longest continuous studies of an outer solar system object.
- Last week, the team hosted a workshop for the microlensing science community to discuss observing strategies for K2's Campaign 9. Planned for April 2016, Campaign 9 will be the first time that a NASA spacecraft will undertake a large-area microlensing survey. Microlensing is a technique that can be used to detect long-orbital period planets, equivalent to our solar system planets like Jupiter and further out Neptune, around distant stars.
Figure 33: From April 26, the fifth campaign of the K2 mission will include observations of nearly 25,000 stars (image credit: NASA Ames and SETI Institute/F. Mullally)
• May 12, 2015: Kepler was launched on March 7, 2009 (UTC). Its mission was to survey a portion of our galaxy to determine what fraction of stars might harbor potentially habitable, Earth-sized exoplanets or planets that orbit other stars. Of particular interest are exoplanets orbiting in the habitable zone — the range of distance from a star in which the surface temperature of an orbiting planet might sustain liquid water. For life as we know it, liquid water is a necessary ingredient. 60)
- Of the more than 1,000 confirmed planets found by Kepler, eight are less than twice Earth-sized and in their stars' habitable zone. All eight orbit stars cooler and smaller than our sun.
- During its four-year prime mission, Kepler simultaneously and continuously measured the brightness of more than 150,000 stars, looking for the telltale dimming that would indicate the presence of an orbiting planet. From these dimmings, or transits, and information about the parent star, researchers can determine a planet's size (radius), the time it takes to orbit its star and the amount of energy received from the host star.
- Kepler's exquisitely precise photometer, or light sensor, was designed to detect minute changes in brightness, to infer the presence of an Earth-sized planet. For a remote observer, Earth transiting the sun would dim its light by less than 1/100th of one percent, or the equivalent of the amount of light blocked by a gnat crawling across a car's headlight viewed from several miles away.
- In May 2014, the Kepler spacecraft began a new mission, K2, to observe parts of the sky along the ecliptic plane, the orbital path of the Earth about the sun, where the familiar constellations of the zodiac lie. This new mission provides scientists with an opportunity to search for even more exoplanets, as well as opportunities to observe notable star clusters, young and old stars, active galaxies and supernovae. The spacecraft continues to collect data in its new mission.
Figure 34: The graphic tells NASA's Kepler spacecraft's story by the numbers from the moment it began hunting for planets outside our solar system on May 12, 2009. From the trove of data collected, we have learned that planets are common, that most sun-like stars have at least one planet and that nature makes planets with unimaginable diversity (image credit: NASA Ames/W Stenzel)
• April 2, 2015: Now in its fourth observing campaign, the Kepler spacecraft continues to operate wonderfully since beginning its new K2 mission in May 2014. Data collected for Campaigns 0, 1 and 2 have been made available to the public through the MAST (Mikulski Archive for Space Telescopes). Campaign 3 data will be processed with a scheduled delivery to MAST in June 2015. 61)
- K2 began its fourth campaign on Feb. 8. The Campaign 4 target set includes nearly 16,000 target stars, which can be searched for exoplanets and examined for an array of astrophysical phenomena. This field includes two notable open star clusters—Pleiades and Hyades, the nearest open cluster to our solar system. Both are located in the constellation of Taurus.
- As expected, the team continues to make improvements in the spacecraft's K2 operations, improving the pointing performance, conserving fuel, extending the observation periods and increasing the number of observed targets. The team currently estimates that the onboard fuel should last until at least December 2017.
Figure 35: From Feb. 7 to April 26, the fourth campaign of the K2 mission will include observations of nearly 16,000 target stars and two notable open star clusters—Pleiades and Hyades (image credit: NASA Ames and SETI Institute/F. Mullally)
• January 6, 2015: How many stars like our sun host planets like our Earth? NASA's Kepler Space Telescope continuously monitored more than 150,000 stars beyond our solar system, and to date has offered scientists an assortment of more than 4,000 candidate planets for further study — the 1,000th of which was recently verified. 62)
- Using Kepler data, scientists reached this milestone after validating that eight more candidates spotted by the planet-hunting telescope are, in fact, planets. The Kepler team also has added another 554 candidates to the roll of potential planets, six of which are near-Earth-size and orbit in the habitable zone of stars similar to our sun. - Three of the newly-validated planets are located in their distant suns' habitable zone, the range of distances from the host star where liquid water might exist on the surface of an orbiting planet. Of the three, two are likely made of rock, like Earth.
- Two of the newly validated planets, Kepler-438b and Kepler-442b, are less than 1.5 times the diameter of Earth. Kepler-438b, 475 light-years away, is 12% bigger than Earth and orbits its star once every 35.2 days. Kepler-442b, 1,100 light-years away, is 33% bigger than Earth and orbits its star once every 112 days. Both Kepler-438b and Kepler-442b orbit stars smaller and cooler than our sun, making the habitable zone closer to their parent star, in the direction of the constellation Lyra. The research paper reporting this finding has been accepted for publication in The Astrophysical Journal.
Figure 36: NASA Kepler's Hall of Fame: Of the more than 1,000 verified planets found by NASA's Kepler Space Telescope, eight are less than twice Earth-size and in their stars' habitable zone. All eight orbit stars cooler and smaller than our sun. The search continues for Earth-size habitable zone worlds around sun-like stars (image credit: NASA)
• December 16, 2014: Kepler and K2 have kept the team very busy over the past couple of months, and we are overdue on providing an update on the great work that's been going on. The spacecraft continues to perform superbly in its two-wheel configuration and is actively collecting data for the K2 mission, while the team has continued to tune the operations to improve the science yield. Meanwhile, we continue analyzing the full four years of Kepler data and delivering the new K2 data to the public at the MAST (Mikulski Archive for Space Telescopes ). 63)
- K2 is now in its seventh month of operation and began its third campaign on Nov. 12. The Campaign 3 FOV (Field of View) includes more than 16,000 target stars, which can be searched for exoplanets and examined for an array of astrophysical information. This campaign also includes observations of a number of objects within our own solar system, including the dwarf planet (225088) 2007 OR10, the largest known body without a name in the solar system, and the planet Neptune and its moon Nereid.
- Campaign 0 data have been delivered to MAST, and Campaign 1 data will follow later this month. Campaign 2 will be processed with a scheduled delivery in February 2015.
- On Oct. 20, the Kepler spacecraft joined the fleet of NASA science assets that observed distant Oort Cloud native Comet Siding Spring as it passed through K2's Campaign 2 FOV on its long journey around the sun. The data collected by K2 will add to the study of the comet, giving scientists an invaluable opportunity to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago.
- While K2 operations proceed, the Kepler team continues work on finalizing the data processing and products for the prime mission. The team is also anticipating another mission milestone: the 1,000th exoplanet discovered by Kepler. To-date Kepler has identified more than 4,000 planet candidates, and 996 have been verified as bona fide planets.
Figure 37: Comet Siding Spring passes through K2's FOV: On Oct. 20, 2014 the Kepler spacecraft joined the fleet of NASA science assets that observed distant Oort Cloud native Comet Siding Spring as it passed through K2's Campaign 2 FOV on its long journey around the sun. The data collected by K2 will add to the study of the comet, giving scientists an invaluable opportunity to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago (image credit: NASA Ames/W Stenzel; SETI Institute/D Caldwell)
Legend to Figure 37: Taken by the Kepler spacecraft operating as the K2 mission, the full-frame image (left) is of the K2 Campaign 2 FOV. The image contains bright stars of the constellation Scorpius, with Antares just on the left edge of module-12 in the middle row second from the left. The globular clusters M4 and NGC 6144 can also be seen on module-12 along with the nebulosity of the well-studied rho-Ophiuchus star-forming region. In the upper right of module-2, the first visible module from the left in the top row, Comet Siding Springs can be seen.
The zoomed image (right) shows the nucleus and fuzzy trailing tail of Comet Siding Spring. Because the comet is moving rapidly, its nucleus is smeared over 20 pixels in this 30-minute exposure image causing it to appear oblong. The tail of the comet extends up and to the left from the nucleus. The comet moved through the K2 field-of-view in about 77 hours. A special purpose aperture was created to capture the comet as it moved through the image, allowing scientists to study its behavior just after the close encounter with Mars.
• September 24, 2014: Astronomers using data from three of NASA's space telescopes — Hubble, Spitzer and Kepler — have discovered clear skies and steamy water vapor on a gaseous planet outside our solar system. The planet is about the size of Neptune, making it the smallest planet from which molecules of any kind have been detected. 64)
- "This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth," said John Grunsfeld, assistant administrator of NASA's Science Mission Directorate in Washington. "Such achievements are only possible today with the combined capabilities of these unique and powerful observatories."
- Clouds in a planet's atmosphere can block the view to underlying molecules that reveal information about the planet's composition and history. Finding clear skies on a Neptune-size planet is a good sign that smaller planets might have similarly good visibility.
- "When astronomers go observing at night with telescopes, they say 'clear skies' to mean good luck," said Jonathan Fraine of the University of Maryland, College Park, lead author of a new study appearing in Nature. 65) "In this case, we found clear skies on a distant planet. That's lucky for us because it means clouds didn't block our view of water molecules."
- The planet, HAT-P-11b, is categorized as an exo-Neptune — a Neptune-sized planet that orbits the star HAT-P-11. It is located 120 light-years away in the constellation Cygnus. This planet orbits closer to its star than does our Neptune, making one lap roughly every five days. It is a warm world thought to have a rocky core and gaseous atmosphere. Not much else was known about the composition of the planet, or other exo-Neptunes like it, until now.
- Part of the challenge in analyzing the atmospheres of planets like this is their size. Larger Jupiter-like planets are easier to see because of their impressive girth and relatively inflated atmospheres. In fact, researchers already have detected water vapor in the atmospheres of those planets. The handful of smaller planets observed previously had proved more difficult to probe partially because they all appeared to be cloudy.
- In the new study, astronomers set out to look at the atmosphere of HAT-P-11b, not knowing if its weather would call for clouds. They used Hubble's Wide Field Camera 3, and a technique called transmission spectroscopy, in which a planet is observed as it crosses in front of its parent star. Starlight filters through the rim of the planet's atmosphere; if molecules like water vapor are present, they absorb some of the starlight, leaving distinct signatures in the light that reaches our telescopes.
- Using this strategy, Hubble was able to detect water vapor in HAT-P-11b. But before the team could celebrate clear skies on the exo-Neptune, they had to show that starspots — cooler "freckles" on the face of stars — were not the real sources of water vapor. Cool starspots on the parent star can contain water vapor that might erroneously appear to be from the planet.
- The team turned to Kepler and Spitzer. Kepler had been observing one patch of sky for years, and HAT-P-11b happens to lie in the field. Those visible-light data were combined with targeted Spitzer observations taken at infrared wavelengths. By comparing these observations, the astronomers figured out that the starspots were too hot to have any steam. It was at that point the team could celebrate detecting water vapor on a world unlike any in our solar system. This discovery indicates the planet did not have clouds blocking the view, a hopeful sign that more cloudless planets can be located and analyzed in the future.
Figure 38: Seeing starlight through a planet's rim: A Neptune-size planet with a clear atmosphere is shown crossing in front of its star in this artist's depiction. Such crossings, or transits, are observed by telescopes like NASA's Hubble and Spitzer to glean information about planets' atmospheres. As starlight passes through a planet's atmosphere, atoms and molecules absorb light at certain wavelengths, blocking it from the telescope's view. The more light a planet blocks, the larger the planet appears. By analyzing the amount of light blocked by the planet at different wavelengths, researchers can determine which molecules make up the atmosphere (image credit: NASA/JPL-Caltech)
Figure 39: A plot of the transmission spectrum for exoplanet HAT-P-11b: With data from NASA's Kepler, Hubble and Spitzer observatories combined, results show a robust detection of water absorption in the Hubble data. Transmission spectra of selected atmospheric models are plotted for comparison (Image Credit: NASA/ESA/STScI, Ref. 64)
• September 2, 2014: The Kepler Mission is exploring the diversity of planets and planetary systems. Its legacy will be a catalog of discoveries sufficient for computing planet occurrence rates as a function of size, orbital period, star type, and insolation flux. The mission has made significant progress toward achieving that goal. Over 3,500 transiting exoplanets have been identified from the analysis of the first 3 y of data, 100 planets of which are in the habitable zone. The catalog has a high reliability rate (85–90% averaged over the period/radius plane), which is improving as follow-up observations continue. Dynamical (e.g., velocimetry and transit timing) and statistical methods have confirmed and characterized hundreds of planets over a large range of sizes and compositions for both single- and multiple-star systems. Population studies suggest that planets abound in our galaxy and that small planets are particularly frequent. Here, I report on the progress Kepler has made measuring the prevalence of exoplanets orbiting within one astronomical unit of their host stars in support of the National Aeronautics and Space Administration's long-term goal of finding habitable environments beyond the solar system. 66) 67)
- Summary: Our blinders to small planets have been lifted, and the exoplanet landscape looks dramatically different from what it did before the launch of NASA's Kepler Mission. A picture is forming in which small planets abound and close-in giants are few, in which the HZs of cool stars are heavily populated with terrestrial planets and the diversity of systems challenges preconceived ideas. The picture will continue to evolve over the next few years as we analyze the remaining data, refine the sample, and quantify the observational biases. Characterization instruments will continue to gain sensitivity ensuring that Kepler's exoplanet discoveries will be studied for years to come. Although Kepler's primary data collection has officially ended, the most significant discovery and analysis phase is underway, enabling the long-term goal of exoplanet exploration: the search for habitable environments and life beyond the solar system.
Figure 40: Non-Kepler exoplanet discoveries (Left) are plotted as mass versus orbital period, colored according to the detection technique. A simplified mass–radius relation is used to transform planetary mass to radius (Right) and the >3,500 Kepler discoveries (yellow) are added for comparison. Eighty-six percent of the non-Kepler discoveries are larger than Neptune, whereas the inverse is true of the Kepler discoveries: 85% are smaller than Neptune (image credit: NASA/ARC, Natalie M. Batalha)
Figure 41: Stellar effective temperature versus insolation (stellar flux at the semimajor axis) for Kepler exoplanets larger than 2 R⊗ (plusses) and smaller than 2 R⊗ (circles). Symbols are colored blue if they lie within the HZ and are sized relative to the Earth (represented by a superimposed image) if they represent a planet smaller than 2 R⊗. The confirmed HZ exoplanets (Kepler-22b, Kepler-62 e and f, Kepler-61b, and Kepler-186f) are displayed as the artist's conceptions (image credit: NASA/ARC, Natalie M. Batalha)
Figure 42: The radius distribution (Left) and period distribution (Right) of planet occurrence rates expressed as the average number of planets per star. The distributions have been marginalized over periods between 0.68 and 50 d (radius distribution) and radii between 0.5 and 22.6 R⊗ (period distribution). H12 refers to ref. 88, F13 refers to ref. 47, and D13 refers to ref. 92. The reported one-sigma uncertainties are shown ((image credit: NASA/ARC, Natalie M. Batalha, Ref. 66)
• August 8, 2014: The K2 mission, the two-wheel operation mode of the Kepler spacecraft conducting observations in the ecliptic, officially began collecting data on May 30. The spacecraft performance has been terrific, and it has remained in fine point throughout the campaign, so far. This first science observation run, called Campaign 1, will collect data for approximately 75 days before concluding in mid-August. K2 is observing more than 12,000 target stars for transiting planets in Campaign 1, and is also observing young and old star clusters, active galactic nuclei and supernovae. In total, more than 21,000 target stars are being observed in Campaign 1. 68)
- The Kepler team has set the K2 target fields, with community input, and the scientific community proposes observation targets through the mission's Guest Observer program. The successful proposals for Campaigns 0 and 1 are available at the Kepler Science Center website, along with their complete target lists. The details for future campaigns will be posted as soon as target management is completed. The next call for proposals for Campaigns 4 and 5 closes on Sept. 23, with an intent to propose due Aug. 8.
- As we continue to learn more about the spacecraft's performance in this operating mode, we expect to see increased performance efficiencies – more targets, less fuel, fewer data interruptions. Meanwhile, we continue to see enthusiastic community response to the observing opportunities. The future observing fields are being locked in early to allow the community time to search the fields and identify the best targets, and in some cases, do pre-campaign, ground-based observing.
- To-date, the Kepler exoplanet search has produced more than 4,200 exoplanet candidates and verified 978 as planets.
Figure 43: K2 Mission science operations timeline (image credit: NASA Ames, M Still)
• July 23, 2014: Thanks to NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the radius of a planet outside our solar system. The size of the exoplanet, dubbed Kepler-93b, is now known to an uncertainty of just 119 km on either side of the planetary body. 69)
- The findings confirm Kepler-93b as a "super-Earth" that is about 1.5 times the size of our planet. Although super-Earths are common in the galaxy, none exist in our solar system. Exoplanets like Kepler-93b are therefore our only laboratories to study this major class of planet.
- With good limits on the sizes and masses of super-Earths, scientists can finally start to theorize about what makes up these weird worlds. Previous measurements, by the Keck Observatory in Hawaii, had put Kepler-93b's mass at about 3.8 times that of Earth. The density of Kepler-93b, derived from its mass and newly obtained radius, indicates the planet is in fact very likely made of iron and rock, like Earth.
- "With Kepler and Spitzer, we've captured the most precise measurement to date of an alien planet's size, which is critical for understanding these far-off worlds," said Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seattle and lead author of a paper on the findings published in the Astrophysical Journal.
• June 2, 2014: Astronomers have discovered a rocky planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. "We were very surprised when we realized what we had found," says astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the analysis using data originally collected by NASA's Kepler space telescope. 70)
- Kepler-10c, as it had been named, had a previously measured size of 2.3 times larger than Earth but its mass was not known until now. The team used the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands to conduct follow-up observations to obtain a mass measurement of the rocky behemoth.
- Worlds such as this were not thought possible to exist. The enormous gravitational force of such a massive body would accrete a gas envelope during formation, ballooning the planet to a gas giant the size of Neptune or even Jupiter. However, this planet is thought to be solid, composed primarily of rock.
- "Just when you think you've got it all figured out, nature gives you a huge surprise – in this case, literally," said Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center in Moffett Field, California. "Isn't science marvelous?"
- Kepler-10c orbits a sun-like star every 45 days, making it too hot to sustain life as we know it. It is located about 560 light-years from Earth in the constellation Draco. The system also hosts Kepler-10b, the first rocky planet discovered in the Kepler data.
Figure 44: An artist's concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed (image credit: Harvard-Smithsonian Center for Astrophysics/David Aguilar)
• May 30, 2014: The Kepler K2 mission reinvigorates the high precision photometry capability of the Kepler Space Telescope, by allowing it to accurately point at target sky fields along the ecliptic (the plane of Earth's orbit). The mission was approved 2014 May 16 and began its Field 1 campaign of observing today (2014 May 30). 71)
Figure 45: This is a photograph of the Milky Way with the approximate locations of Kepler K2 campaign target fields, including the Field 1 campaign which the Kepler Space Telescope began observing on May 30, 2014 (image credit: ESO/S. Brunier/NASA Kepler Mission/Wendy Stenzel)
• May 16, 2014: The team received good news from NASA HQ — the K2 (Kepler Second Light) mission, the two-wheel operation mode of the Kepler spacecraft observing in the ecliptic, has been approved based on a recommendation from the agency's 2014 Senior Review of its operating missions. 72)
- The approval provides two years of funding for the K2 mission to continue exoplanet discovery, and introduces new scientific observation opportunities to observe notable star clusters, young and old stars, active galaxies and supernovae.
- After the second wheel of Kepler's guidance control system failed last year during the spacecraft's extended mission, engineers devised a clever solution to manage the sun's radiation pressure and limit its effect on the spacecraft pointing. K2 will observe target fields along the ecliptic plane, the orbital path of planets in our solar system also know as the zodiac, for approximately 75-day campaigns.
- The team is currently finishing up an end-to-end shakedown of this approach with a full-length campaign (Campaign 0), and is preparing for Campaign 1, the first K2 science observation run, scheduled to begin May 30.
• April 17, 2014: Using NASA's Kepler Space Telescope, astronomers have discovered the first Earth-size planet orbiting a star in the "habitable zone" — the range of distance from a star where liquid water might pool on the surface of an orbiting planet. The discovery of Kepler-186f confirms that planets the size of Earth exist in the habitable zone of stars other than our sun. 73)
- While planets have previously been found in the habitable zone, they are all at least 40 percent larger in size than Earth and understanding their makeup is challenging. Kepler-186f is more reminiscent of Earth. "The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington. "Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds."
- Although the size of Kepler-186f is known, its mass and composition are not. Previous research, however, suggests that a planet the size of Kepler-186f is likely to be rocky. "We know of just one planet where life exists — Earth. When we search for life outside our solar system we focus on finding planets with characteristics that mimic that of Earth," said Elisa Quintana, research scientist at the SETI Institute at NASA's Ames Research Center in Moffett Field, Calif., and lead author of the paper published today in the journal Science. 74) "Finding a habitable zone planet comparable to Earth in size is a major step forward."
- Kepler-186f resides in the Kepler-186 system, about 500 light-years from Earth in the constellation Cygnus. The system is also home to four companion planets, which orbit a star half the size and mass of our sun. The star is classified as an M dwarf, or red dwarf, a class of stars that makes up 70 percent of the stars in the Milky Way galaxy.
- Kepler-186f orbits its star once every 130-days and receives one-third the energy from its star that Earth gets from the sun, placing it nearer the outer edge of the habitable zone. On the surface of Kepler-186f, the brightness of its star at high noon is only as bright as our sun appears to us about an hour before sunset. "Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has," said Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at Ames, and co-author of the paper. "Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth."
Figure 46: Kepler-186f resides in the Kepler-186 system about 500 light-years from Earth in the constellation Cygnus. The system is also home to four inner planets, seen lined up in orbit around a host star that is half the size and mass of the sun (image credit: NASA Ames/SETI Institute/JPL-Caltech)
Figure 47: Kepler-186 and the Solar System (image credit: NASA Ames/SETI Institute/JPL-Caltech)
Legend to Figure 47: The diagram compares the planets of our inner solar system to Kepler-186, a five-planet star system about 500 light-years from Earth in the constellation Cygnus. The five planets of Kepler-186 orbit an M dwarf, a star that is is half the size and mass of the sun.
• February 25, 2014: Preparations continue for the first K2 campaign in the ecliptic plane, the orbital path of planets in our solar system. Scheduled to begin in early March, Campaign 0 primarily will be an engineering dress rehearsal for the K2 mission. This initial campaign will ensure that the K2 mission will be ready to proceed if it is approved, following the 2014 Astrophysics Senior Review of Operating Missions. 75)
- During the final test prior to this campaign we were very encouraged to see that the spacecraft operated throughout the test using the fine guidance sensors mounted on the focal plane. Having now brought the data back from the spacecraft, we have found that during the test, one of the science detector modules failed.
- The Kepler focal plane is made up of a mosaic of 21 science detector modules (Figure 11). Four years ago, less than a year into the mission, one of the modules (Module 3) failed. An extensive review was unable to determine a specific cause, but was able to isolate the problem to a part failure in the circuitry powering that module. The new failure, Module 7, appears to be another occurrence of the same, or very similar, problem. We have only begun our assessment of the problem, but it is likely to be another isolated occurrence of a part failure, with no overall implications to a potential K2 mission. The remaining 19 modules still allow for a very large view of the sky, and the target resources that would have fallen on Module 7 have been reassigned to the remaining modules. At this time, it does not appear that this will have any impact on the Campaign 0 planning.
- In the mean time, a paper has been submitted to the astronomy journal Publications of the Astronomical Society of the Pacific (PASP) describing details of the proposed K2 operations and the results of the testing that have been accomplished to characterize the mission performance. The paper describes the potential science that the new mission could deliver, and is intended to inform the scientific community of the potential opportunities so, if approved, the mission would be ready to effectively conduct meaningful studies. 76)
• December 4, 2013: Based on an independent science and technical review of the Kepler project's concept for a Kepler two-wheel mission extension, Paul Hertz, NASA's Astrophysics Division director, has decided to invite Kepler to the Senior Review for astrophysics operating missions in early 2014. 77)
- The Kepler team's proposal, dubbed K2, demonstrated a clever and feasible methodology for accurately controlling the Kepler spacecraft at the level of precision required for scientifically valuable data collection. The team must now further validate the concept and submit a Senior Review proposal that requests the funding necessary to continue the Kepler mission, with sufficient scientific justification to make it a viable option for the use of NASA's limited resources.
• November 25, 2013: A repurposed Kepler Space Telescope may soon start searching the sky again. A new mission concept, dubbed K2, would continue Kepler's search for other worlds, and introduce new opportunities to observe star clusters, young and old stars, active galaxies and supernovae. 78)
- With the failure of a second reaction wheel, the spacecraft can no longer precisely point at the mission's original field of view. The culprit is none other than our own sun. The very body that provides Kepler with its energy needs also pushes the spacecraft around by the pressure exerted when the photons of sunlight strike the spacecraft. Without a third wheel to help counteract the solar pressure, the spacecraft's ultra-precise pointing capability cannot be controlled in all directions.
- However, Kepler mission and Ball Aerospace engineers have developed an innovative way of recovering pointing stability by maneuvering the spacecraft so that the solar pressure is evenly distributed across the surfaces of the spacecraft. To achieve this level of stability, the orientation of the spacecraft must be nearly parallel to its orbital path around the sun, which is slightly offset from the ecliptic, the orbital plane of Earth. The ecliptic plane defines the band of sky in which lie the constellations of the zodiac.
- This technique of using the sun as the 'third wheel' to control pointing is currently being tested on the spacecraft and early results are already coming in. During a pointing performance test in late October, a full frame image of the space telescope's full field of view was captured showing part of the constellation Sagittarius. - Photons of light from a distant star field were collected over a 30-minute period and produced an image quality within five percent of the primary mission image quality, which used four reaction wheels to control pointing stability. Additional testing is underway to demonstrate the ability to maintain this level of pointing control for days and weeks.
- To capture the telltale signature of a distant planet as it crosses the face of its host star and temporarily blocks the amount of starlight collected by Kepler, the spacecraft must maintain pointing stability over these longer periods."This 'second light' image provides a successful first step in a process that may yet result in new observations and continued discoveries from the Kepler space telescope," said Charlie Sobeck, Kepler deputy project manager at NASA/ARC in Moffett Field, CA.
- The K2 (Kepler Second Light) mission concept has been presented to NASA Headquarters. A decision to proceed to the 2014 Senior Review – a biennial assessment of operating missions – and propose for budget to fly K2 is expected by the end of 2013.
- Kepler's original mission, which is still in progress to fully process the wealth of data collected, is to determine what percentage of stars like the sun harbor small planets the approximate size and surface temperature of Earth. For four years, the space telescope simultaneously and continuously monitored the brightness of more than 150,000 stars, recording a measurement every 30 minutes. - More than a year of the data collected by Kepler remains to be fully reviewed and analyzed.
Figure 48: This concept illustration depicts how solar pressure can be used to balance NASA's Kepler spacecraft, keeping the telescope stable enough to continue searching for transiting planets around distant stars (image credit: NASA Ames/W Stenzel)
• August 15, 2013: Following months of analysis and testing, the Kepler Space Telescope team is ending its attempts to restore the spacecraft to full working order, and now is considering what new science research it can carry out in its current condition. Two of Kepler's four gyroscope-like reaction wheels, which are used to precisely point the spacecraft, have failed. The first was lost in July 2012, and the second in May 2013. Engineers' efforts to restore at least one of the wheels have been unsuccessful. 79)
- Kepler completed its prime mission in November 2012 and began its four-year extended mission at that time. However, the spacecraft needs three functioning wheels to continue its search for Earth-sized exoplanets, which are planets outside our solar system, orbiting stars like our sun in what's known as the habitable zone — the range of distances from a star where the surface temperature of a planet might be suitable for liquid water. As scientists analyze previously collected data, the Kepler team also is looking into whether the space telescope can conduct a different type of science program, potentially including an exoplanet search, using the remaining two good reaction wheels and thrusters.
- "Kepler has made extraordinary discoveries in finding exoplanets including several super-Earths in the habitable zone," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington. "Knowing that Kepler has successfully collected all the data from its prime mission, I am confident that more amazing discoveries are on the horizon."
- On Aug. 8, engineers conducted a system-level performance test to evaluate Kepler's current capabilities. They determined wheel 2, which failed last year, can no longer provide the precision pointing necessary for science data collection. The spacecraft was returned to itsPRS (Point Rest State), which is a stable configuration where Kepler uses thrusters to control its pointing with minimal fuel use.
- An engineering study will be conducted on the modifications required to manage science operations with the spacecraft using a combination of its remaining two good reaction wheels and thrusters for spacecraft attitude control. - Informed by contributions from the broader science community in response to the call for scientific white papers announced Aug. 2, the Kepler project team will perform a study to identify possible science opportunities for a two-wheel Kepler mission.
- Depending on the outcome of these studies, which are expected to be completed later this year, NASA will assess the scientific priority of a two-wheel Kepler mission. Such an assessment may include prioritization relative to other NASA astrophysics missions competing for operational funding at the NASA Senior Review board early next year.
- From the data collected in the first half of its mission, Kepler has confirmed 135 exoplanets and identified over 3,500 candidates. The team continues to analyze all four years of collected data, expecting hundreds, if not thousands, of new discoveries including the long-awaited Earth-size planets in the habitable zone of sun-like stars. Though the spacecraft will no longer operate with its unparalleled precision pointing, scientists expect Kepler's most interesting discoveries are still to come.
• July 24, 2013: On July 18, 2013 the operations team initiated exploratory recovery tests on the spacecraft's two failed wheels. The recovery tests are a series of steps to characterize the performance of Reaction Wheels (RW) 4 and 2, and to determine if either could be returned to operation. In response to test commands, wheel 4 did not spin in the positive (or clockwise) direction but the wheel did spin in the negative (or counterclockwise) direction. Wheel 4 is thought to be the more seriously damaged of the two. - On July 22, the team proceeded with a test of RW2. Wheel 2 responded to test commands and spun in both directions. 80)
- Over the next two weeks, engineers will review the data from these tests and consider what steps to take next. Although both wheels have shown motion, the friction levels will be critical in future considerations. The details of the wheel friction are under analysis.
- Kepler requires extremely precise pointing to detect the faint periodic dimming of distant starlight— the telltale sign of a planet transiting the face of its host star. Too much friction from the reaction wheels can cause vibration and impact the pointing precision of the telescope.
• May 21, 2013: Following the apparent failure of reaction wheel 4 on May 11, 2013, engineers were successful at transitioning the spacecraft from a Thruster-Controlled Safe Mode to PRS (Point Rest State) at approximately 3:30 p.m. PDT (Pacific Daylight Time) on Wednesday, May 15, 2013. The spacecraft has remained safe and stable in this attitude and is no longer considered to be in a critical situation. 81)
- As part of a normal spacecraft response to a pointing error, redundant electronics were automatically powered off to isolate them as a possible cause. However, once the team recovered the spacecraft to PRS and exonerated those systems, they were turned back on, providing full redundancy to the spacecraft. The reaction wheels remain offline. The photometer, which was turned off to reduce the power load, will be turned back on in the near future to keep thermal conditions of the spacecraft within nominal operating parameters. Kepler is not in science data collection.
- PRS was developed in order to preserve fuel for an eventual recovery effort once a second wheel failed. This state uses thrusters to control the pointing of the spacecraft, tipping it towards the sun and letting the solar pressure tip it back away, resembling the motion of a pendulum. This is a very fuel-efficient mode, and it also provides an on-demand telemetry link to allow engineers to monitor and command the spacecraft. With nearly a week of PRS operations, the fuel usage appears to be on the low end of our estimates, allowing time for recovery planning.
Figure 49: This diagram of the Kepler spacecraft shows the location of two of the four reaction wheels that control the pointing accuracy of the vehicle. Reaction wheel 4 failed on May 11, 2013 (image credit: Ball Aerospace)
• May 15, 2013: At our semi-weekly contact on Tuesday, May 14, 2013, we found the Kepler spacecraft once again in safe mode. As was the case earlier this month, this was a Thruster-Controlled Safe Mode. The root cause is not yet known, however the proximate cause appears to be an attitude error. The spacecraft was oriented with the solar panels facing the sun, slowly spinning about the sun-line. The communication link comes and goes as the spacecraft spins. 82)
- We attempted to return to reaction wheel control as the spacecraft rotated into communication, and commanded a stop rotation. Initially, it appeared that all three wheels responded and that rotation had been successfully stopped, but reaction wheel 4 remained at full torque while the spin rate dropped to zero. This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing. The spacecraft was then transitioned back to Thruster-Controlled Safe Mode.
- An Anomaly Review Board concurred that the data appear to unambiguously indicate a wheel 4 failure, and that the team's priority is to complete preparations to enter Point Rest State. Point Rest State is a loosely-pointed, thruster-controlled state that minimizes fuels usage while providing a continuous X-band communication downlink. The software to execute that state was loaded to the spacecraft last week, and last night the team completed the upload of the parameters the software will use.
- The spacecraft is stable and safe, if still burning fuel. Our fuel budget is sufficient that we can take due caution while we finish our planning. In its current mode, our fuel will last for several months. Point Rest State would extend that period to years.
- We have requested and received additional NASA Deep Space Network communication coverage, and this morning the Anomaly Review Board approved the transition to Point Rest State later today. Because this is a new operating mode of the spacecraft, the team will closely monitor the spacecraft, but no other immediate actions are planned. We will take the next several days and weeks to assess our options and develop new command products. These options are likely to include steps to attempt to recover wheel functionality and to investigate the utility of a hybrid mode, using both wheels and thrusters.
- With the failure of a second reaction wheel, it's unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry. However, no decision has been made to end data collection.
• May 9, 2013: During a scheduled semi-weekly contact on Friday, May 3, 2013, engineers discovered that the Kepler spacecraft was in a self-protective state called a safe mode. The spacecraft was returned to science data collection just before midnight on Monday, May 6, 2013. 83)
- The spacecraft entered thruster-controlled safe mode at about 7:30 p.m. PDT on Wednesday, May 1, 2013. The recovery operation began at about 5 p.m. PDT on Friday, May 3, 2013, after engineers had verified that the spacecraft was otherwise operating normally. The spacecraft responded well to commands and transitioned from thruster control to reaction wheel control as planned.
- Following the safe mode recovery, the team performed a routine monthly data downlink from the on-board solid-state recorder on May 5, 2013, and returned to science data collection. The monthly download was originally scheduled for May 8-9, 2013.
- The root cause of the safe mode is not yet known but the engineering team is analyzing the data set downloaded during the monthly contact. The reaction wheels do not appear to be the cause of the safe mode. Early indications suggest anomalous star tracker performance. The loss of science data is estimated to be about five days.
• April 18, 2013: NASA's Kepler mission has discovered two new planetary systems that include three super-Earth-size planets in the "habitable zone," the range of distance from a star where the surface temperature of an orbiting planet might be suitable for liquid water. The Kepler-62 system has five planets; 62b, 62c, 62d, 62e and 62f. The Kepler-69 system has two planets; 69b and 69c. Kepler-62e, 62f and 69c are the super-Earth-sized planets. 84)
- Two of the newly discovered planets orbit a star smaller and cooler than the sun. Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62f is likely to have a rocky composition. Kepler-62e, orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth.
- The third planet, Kepler-69c, is 70 percent larger than the size of Earth, and orbits in the habitable zone of a star similar to our sun. Astronomers are uncertain about the composition of Kepler-69c, but its orbit of 242 days around a sun-like star resembles that of our neighboring planet Venus.
- Scientists do not know whether life could exist on the newfound planets, but their discovery signals we are another step closer to finding a world similar to Earth around a star like our sun.
Figure 50: Relative sizes of all of the habitable-zone planets discovered to date alongside Earth. Left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and Earth (except for Earth, these are artists' renditions), image credit: NASA Ames/JPL-Caltech
• March 6, 2013: Since returning to science data collection on Jan. 27, 2013, after a 10-day precautionary wheel rest safe mode, the spacecraft has been performing well and continues to make science observations. The next monthly data download from the spacecraft is planned for March 6-7. 85)
- The team continues to monitor the health of the spacecraft and reaction wheel friction levels during semi-weekly contacts using NASA's Deep Space Network. Preliminary indications suggest that reaction wheel #4 continues to exhibit higher levels of friction after the wheel rest operation. During the March monthly download, high rate data of the wheel friction levels will be returned from the on-board recorder and the team will be able to perform a more thorough analysis.
- Communications with Kepler during the semi-weekly contacts is accomplished using the spacecraft's LGA (Low-Gain Antenna) that operates on an X-band frequency. Low-rate engineering data is brought down through the LGA while the spacecraft's orientation remains fixed on the target stars. The collection of science data is not interrupted for semi-weekly contacts. Once a month, a complete science and engineering data set is downloaded from the on-board solid-state recorder. During these monthly downloads, the spacecraft is turned away from its target stars to point the HGA (High-Gain Antenna) at Earth, temporarily interrupting the scientific observations. These downloads are thus able to return data at a much higher rate on a Ka-band frequency. The monthly downloads are typically done at a downlink rate of 4.3 Mbit/s, while the semi-weekly contacts can drop to data rates as low as 100 bit/s, depending on range and spacecraft orientation.
• January 22, 2013: Earlier this month during a semi-weekly contact with the spacecraft, the team detected an increase in the amount of torque required to spin one of the three remaining reaction wheels. This increase in friction occurred before the Jan. 11, 2013 quarterly roll, and persisted after the spacecraft roll and several momentum desaturations of the reaction wheels. Increased friction over a prolonged period can lead to accumulated wear on the reaction wheel, and possible wheel failure. To minimize wheel friction, the team implemented several mitigations including increased operating temperatures, higher spin rates, and bi-directional operation following the failure of reaction wheel #2 in July 2012. 86)
- Given the persistence of this recent event in reaction wheel #4, the project team will place the Kepler spacecraft in a "wheel rest" safe mode for a period of ten days beginning today. Science data collection will be stopped during this period and the spacecraft solar panel orientation will be aligned with the sun to maintain positive power for Kepler. This is similar to a normal safe mode configuration, but with thrusters maintaining attitude instead of reaction wheels. Resting the wheels provides an opportunity to redistribute internal lubricant, potentially returning the friction to normal levels.
- Once the 10-day rest period ends, the team will recover the spacecraft from this resting safe mode and return to science operations. That is expected to take approximately three days.
Figure 51: Photo of a Kepler reaction wheel (image credit: BATC)
• November 14, 2012: NASA is marking two milestones in the search for planets like Earth; the successful completion of the Kepler Space Telescope's 3 1/2- year prime mission and the beginning of an extended mission that could last as long as four years. 87)
- Scientists have used Kepler data to identify more than 2,300 planet candidates and confirm more than 100 planets. Kepler is teaching us the galaxy is teeming with planetary systems and planets are prolific, and giving us hints that nature makes small planets efficiently. So far, hundreds of Earth-size planet candidates have been found as well as candidates that orbit in the habitable zone, the region in a planetary system where liquid water might exist on the surface of a planet. None of the candidates is exactly like Earth. With the completion of the prime mission, Kepler now has collected enough data to begin finding true sun-Earth analogs — Earth-size planets with a one-year orbit around stars similar to the sun.
- "The initial discoveries of the Kepler mission indicate at least a third of the stars have planets and the number of planets in our galaxy must number in the billions," said William Borucki, Kepler principal investigator at NASA/ARC in Moffett Field, CA. "The planets of greatest interest are other Earths and these could already be in the data awaiting analysis. Kepler's most exciting results are yet to come."
• August 22, 2012: Two newly submitted studies verify 41 new transiting planets in 20 star systems. These results may increase the number of Kepler's confirmed planets by more than 50 percent: to 116 planets hosted in 67 systems, over half of which contain more than one planet. The papers are currently under scientific peer-review. Nineteen of the newly validated planetary systems have two closely spaced transiting planets and one system has three. Five of the systems are common to both of these independent studies. The planets range from Earth-size to more than seven times the radius of Earth, but generally orbit so close to their parent stars that they are hot, inhospitable worlds. The planets were confirmed by analyzing TTVs (Transit Timing Variations). In closely packed systems, the gravitational pull of the planets causes the acceleration or deceleration of a planet along its orbit. 88) 89)
- These "tugs" cause the orbital period of each planet to change from one orbit to the next. TTV demonstrates that two transiting planet candidates are in the same system and that their masses are planetary in nature. "These systems, with their large gravitational interactions, give us important clues about how planetary systems form and evolve," said lead researcher Jason Steffen, the Brinson postdoctoral fellow at Fermilab Center for Particle Astrophysics in Batavia, Ill. "This information helps us understand how our own solar system fits into the population of all planetary systems." The two research teams used data from NASA's Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets. "The sheer volume of planet candidates being identified by Kepler is inspiring teams to look at the planet confirmation and characterization process differently. This TTV confirmation technique can be applied to large numbers of systems relatively quickly and with little or no follow-up observations from the ground," said Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center, Moffett Field, Calif. "Perhaps the bottleneck between identifying planet candidates and confirming them just got a little wider."
Figure 52: The diagram shows the newly submitted transiting planets in green along with the unconfirmed planet candidates in the same system in violet. The systems are ordered horizontally by increasing Kepler number and KOI (Kepler Object of Interest) designation and vertically by orbital period (image credit: Jason Steffen, Fermilab Center for Particle Astrophysics)
• April 4, 2012: NASA's Kepler mission has been approved for extension through fiscal year 2016 based on a recommendation from the Agency's Senior Review of its operating missions. The extension provides four additional years to find Earth-size planets in the habitable zone — the region in a planetary system where liquid water could exist on the surface of an orbiting planet – around sun-like stars in our galaxy. 90)
- "Kepler has revolutionized our understanding of exoplanets and the study of stellar seismology and variability," said Roger Hunter, Kepler project manager at NASA Ames Research Center, Moffett Field, Calif. "There is currently no other mission in development that can replace or surpass the precision of Kepler. This extended mission will afford Kepler a unique opportunity to rewrite our understanding of the galaxy and our place in it."
- The mission's discoveries beyond our solar system include the first unquestionably rocky planet; the first multiple-transiting planet system; the first small planet in the habitable zone; the first Earth-size planets; the smallest Mars-size planets; and the confirmation of a new class of double-star planetary systems.
• February 28, 2012: 1,091 new transiting planet candidates have emerged from analysis of Kepler spacecraft data spanning May 2009 to September 2010, bringing the total count to 2,321 Kepler planet candidates orbiting 1,790 host stars. A clear trend toward smaller planets at longer orbital periods is evident with each new catalog release. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant. 91)
- The largest increases in planet candidates in the current release are for the smallest ones. The cumulative catalog now contains over 200 Earth-size planet candidates and more than 900 that are smaller than twice Earth-size (super-Earths), a 197% increase (compared to a 52% increase in number of candidates larger than 2 Earth radii).
- There is 123% increase in planet candidates with orbital periods (time taken to orbit the star) greater than of 50 days versus 85% for candidate periods less than 50 days. Of the 46 planet candidates found in the habitable zone (where liquid water could exist), 10 are near-Earth-size. The gains in smaller size and longer period candidates are larger than expected and indicate significant improvements the Kepler data analysis software.
Figure 53: Kepler mission statistic of planet candidate sizes (image credit: NASA)
• January 26, 2012: NASA's Kepler mission has discovered 11 new planetary systems hosting 26 confirmed planets. These discoveries nearly double the number of verified planets and triple the number of stars known to have more than one planet that transits, or passes in front of, the star. Such systems will help astronomers better understand how planets form. 92) 93)
- The planets orbit close to their host stars and range in size from 1.5 times the radius of Earth to larger than Jupiter. Fifteen are between Earth and Neptune in size. Further observations will be required to determine which are rocky like Earth and which have thick gaseous atmospheres like Neptune. The planets orbit their host star once every six to 143 days. All are closer to their host star than Venus is to our sun.
- "Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky," said Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington. "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."
- Each of the planetary systems contains two to five closely spaced transiting planets. In tightly packed planetary systems, the gravitational pull of the planets on each other causes some planets to accelerate and some to decelerate along their orbits. The acceleration causes the orbital period of each planet to change. Kepler detects this effect by measuring the change, or so-called TTVs (Transit Timing Variations).
Figure 54: The artist's rendering depicts the multiple planet systems discovered by NASA's Kepler mission. Out of hundreds of candidate planetary systems, scientists had previously verified six systems with multiple transiting planets (denoted here in red). Now, Kepler observations have verified planets (shown here in green) in 11 new planetary systems. Many of these systems contain additional planet candidates that are yet to be verified (shown here in dark purple). For reference, the eight planets of our Solar System are shown in blue along the left edge of the image (image credit: NASA Ames/Jason Steffen, Fermilab Center for Particle Astrophysics)
• January 11, 2012: Using data from NASA's Kepler Mission, astronomers announced the discovery of two new transiting "circumbinary" planet systems – planets that orbit two stars. This work establishes that such "two sun" planets are not rare exceptions, but are in fact common with many millions existing in our Galaxy. The work was published on-line today in the journal Nature and was presented by Dr. William Welsh of San Diego State University at the American Astronomical Society meeting in Austin, TX on behalf of the Kepler Science Team. 94) 95)
- The two new planets, named Kepler-34 b and Kepler-35 b, are both gaseous Saturn-size planets. Kepler-34 b orbits its two Sun-like stars every 289 days, and the stars themselves orbit and eclipse each other every 28 days. The eclipses allow a very precise determination of the stars' sizes. Kepler-35 b revolves about a pair of smaller stars (80 and 89 percent of the Sun's mass) every 131 days, and the stars orbit and eclipse one another every 21 days. Both systems reside in the constellation Cygnus, with Kepler-34 at 4900 light-years from Earth, and Kepler-35 at 5400 light-years, making these among the most distant planets discovered.
- While long anticipated in both science and science fiction, the existence of a circumbinary planet orbiting a pair of normal stars was not definitively established until the discovery of Kepler-16 b, announced by the Kepler Team last September. Like Kepler-16 b, these new planets also transit (eclipse) their host stars, making their existence unambiguous. When only Kepler-16 b was known, many questions remained about the nature of circumbinary planets – what kinds of orbits, masses, radii, temperatures, etc., could they have? And most of all, was Kepler-16 b just a fluke? With the discovery of Kepler-34 b and 35 b, astronomers can now answer many of those questions and begin to study an entirely new class of planets. "It was once believed that the environment around a pair of stars would be too chaotic for a circumbinary planet to form, but now that we have confirmed three such planets, we know that it is possible, if not probable, that there are at least millions in the Galaxy," said Welsh, who led the team of 46 investigators involved in this research.
- The discovery was made possible by the three unique capabilities of the Kepler space telescope: its ultra-high precision, its ability to simultaneously observe roughly 160,000 stars, and its long- duration near-continuous measurements of the brightness of stars. Additional work using ground- based telescopes provided velocity measurements of the stars needed to confirm that these candidates are really planets. "The search is on for more circumbinary planets," said co-author Dr. Joshua Carter of the Harvard-Smithsonian Center for Astrophysics, "and we hope to use Kepler for years to come."
Figure 55: This artful rendition of the Kepler-35 planet system, in which a Saturn-size planet orbits a pair of stars. The larger star is similar to the size of the Sun, while the smaller star is 79% of the Sun's radius. The stars orbit and eclipse each other every 21 days, but the eclipses do not occur exactly periodically. This variation in the times of the eclipses motivated the search for the planet, which was discovered to transit the stars as it orbits the pair every 131 days. Analogous events led to the discovery of the planet Kepler-34. The discovery of these two new systems establishes a new class of circumbinary' planets, and suggests there are many millions of such giant planets in our Galaxy (image credit: Lynette Cook)
• August 11, 2011: Astronomers have discovered the darkest known exoplanet - a distant, Jupiter-sized gas giant known as TrES-2b. Their measurements show that TrES-2b reflects less than one percent of the sunlight falling on it, making it blacker than coal or any planet or moon in our solar system. "TrES-2b is considerably less reflective than black acrylic paint, so it's truly an alien world," said astronomer David Kipping of the Harvard-Smithsonian CfA (Center for Astrophysics), lead author on the paper reporting the research. 96)
- The team monitored the brightness of the TrES-2 system as the planet orbited its star. They detected a subtle dimming and brightening due to the planet's changing phase."By combining the impressive precision from Kepler with observations of over 50 orbits, we detected the smallest-ever change in brightness from an exoplanet: just 6 parts per million," said Kipping. "In other words, Kepler was able to directly detect visible light coming from the planet itself." The extremely small fluctuations proved that TrES-2b is incredibly dark. A more reflective world would have shown larger brightness variations as its phase changed.
Figure 56: The distant exoplanet TrES-2b, shown here in an artist's conception, is darker than the blackest coal. This Jupiter-sized world reflects less than one percent of the light that falls on it, making it blacker than any planet or moon in our solar system. Astronomers aren't sure what vapors in the planet's superheated atmosphere cloak it so effectively (image credit: David A. Aguilar, CfA)
• April 12, 2011: The scientific investigation of sun-like stars has taken a major step forward thanks to the Kepler Mission. In addition to searching for exoplanets, it is providing exquisite data on stellar oscillations. "The sound inside the stars makes them ring or vibrate like musical instruments," said Bill Chaplin from the University of Birmingham's School of Physics and Astronomy, the lead author of this paper. "If you measure the pitch of the notes produced by an instrument it can tell you how big the instrument is. The bigger the instrument is, the lower the pitch and deeper the sound. This is how we can tell how big a star is - from its stellar music." 97) 98)
- Oscillation measurements are used to accurately determine fundamental stellar properties like mass, size, and age. This is where theory meets observation. Scientists can synthesize a snapshot of our galaxy and all the stars it contains using models based on everything we know about how much raw material there is in our galaxy for building stars, what types of stars are made, how they evolve with time, and how long they live. They can then compare the properties of stars in this synthetic snapshot with the properties of the sun-like stars in the asteroseismic survey. In essence, the team has taken a census and compared it to predictions, and found that the sizes of the stars are consistent with the predictions, but the masses are not. The asteroseismic survey suggests that the number of low mass stars is slightly larger than expected. This work sends theoreticians back to refine their models and will ultimately lead to a better understanding of the structure and evolution of stars in our galaxy.
- "Before Kepler we had asteroseismic data on only about 20 such stars - We now have an orchestra of stars to play with," said Hans Kjeldsen from Aarhus from the Danish Asteroseismology Center in Aarhus, who coordinates KASC (Kepler Asteroseismic Science Consortium). "This opens up huge possibilities for probing stellar evolution and obtaining a clearer picture of the past and future of our own sun and how our galaxy, and others like it, has evolved over time. We can, for example, pick out stars that have the same mass of the sun but have different ages, to, in effect, follow the sun in time."
• April 7, 2011: While the quest for Earth-like planets around other stars using the NASA's Kepler space telescope has recently produced many exciting discoveries, other branches of stellar astrophysics also benefit from the ultraprecise space photometry offered by the revolutionary Kepler satellite. An international group of European, Australian and American researchers report on the discovery of a unique stellar system in a paper accepted for publication in the Science magazine. The object, catalogued as HD 181068 and known as Trinity' within the authorship team, is a 7th magnitude star that is almost visible to the naked eye, and the seemingly single star is in reality a complex triple system in which three stars reside in a very special geometry, showing mutual eclipses as each of the stars gets behind or in front of the others. The most luminous object is a red giant star around which a close pair of two red dwarfs orbits with a period of 45,5 days. 99) 100)
- "Thanks to the fortunate viewing angle from Earth, the combined light from the three stars change very characteristically: there are sharp brightness decreases with a period of 0.9 days produced by the mutual eclipses of the close pair of dwarfs, while it takes 2 days for the close pair to pass in front of or behind the red giant" - says Aliz Derekas (Eotvos University and Konkoly Observatory, Budapest, Hungary), the lead author of the paper. A mind-boggling feature of the variations is that when the red dwarfs are in front of the red giant, their short-period eclipses disappear. This is because the surface brightnesses of the three stars are actually very similar, and just as a white rabbit cannot be seen in snow-fall, the red dwarfs in front of the red giant are also almost invisible, hence no light is lost when they eclipse each other.
Figure 57: Relative sizes of HD181068. The artist's rendering compares the approximate size and color of the stars in the triple-eclipsing system HD 181068 (image credit: NASA)
- The authors discovered this interesting system in June 2010 and consequently took ground-based observations. "The spectroscopic measurements revealed the periodic motion of the largest star in the system with the wide orbital period of 45,5 days. The 2-day long eclipses are so similar at first sight that we thought that the outer orbital period is 22.7 days. Only after having obtained the whole radial velocity curve, we realized the tiny differences in the long period minima and that the real period was the double of it.'- says Laszlo Kiss (Konkoly Observatory), the second author of the discovery paper. Further observations using interferometry were used to measure the angular size of the red giant. "Combining the angular diameter with the known distance of the system we were able to measure the absolute radius of the red giant, which was a great achievement given its large distance of 800 light years" - adds Daniel Huber (University of Sydney, Australia), who led the interferometric observations using the Center for High-Angular Resolution Astronomy (CHARA) at Mount Wilson Observatory in California, USA. The results show that the largest star in the system is 12.4 times larger than our Sun. The scientists could also estimate the mass of the main component as 3 times that of the Sun.
- The discovery of this complex system is significant because HD 181068 is a real astrophysical laboratory where changes in the orbital elements, unlike in the usual cases in astronomy, can be detected in a few years from now, i.e. we can compare theoretical predictions and observed changes on human timescale. In addition, HD 181068 has further peculiar features. Careful analyses of red giant stars observed by Kepler have shown that all red giant stars should exhibit oscillations similar to those in the Sun. The frequency of these oscillations can be theoretically determined knowing the basic physical parameters of the red giant (mass, temperature, radius). However, there is no sign of such oscillations in the red giant component of HD 181068 which means there must be a mysterious mechanism that suppresses the pulsation. "Surprisingly, we do detect some variability but with periods that are closely linked to the orbital period of the close pair in the system" according to Dr. Derekas. This may indicate that tidal forces of the close pair induce vibes in the surface of the red giant.
• Feb. 1, 2011: NASA's Kepler mission has discovered its first Earth-size planet candidates and its first candidates in the habitable zone, a region where liquid water could exist on a planet's surface. Five are both near Earth-size and orbit in the habitable zone of their stars. The discoveries are part of several hundred new planet candidates identified in new Kepler mission science data, released on Tuesday, Feb. 1. The findings increase the number of planet candidates identified by Kepler to-date to 1,235. 170 stars show evidence of multiple planetary candidates. Kepler-11 has the most confirmed transiting planets ever discovered and all six of its planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury's. 101) 102)
- The findings are based on the results of observations conducted May 12 to Sept. 17, 2009, of more than 156,000 stars in Kepler's field of view, which covers approximately 1/400 of the sky. "The fact that we've found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting sun-like stars in our galaxy," said William Borucki of NASA's Ames Research Center in Moffett Field, Calif., the mission's science principal investigator. "We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone, some of which could have moons with liquid water." 103)
- Among the stars with planetary candidates, 170 show evidence of multiple planetary candidates. Kepler-11, located approximately 2,000 light years from Earth, is the most tightly packed planetary system yet discovered. All six of its confirmed planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury's. The only other star with more than one confirmed transiting planet is Kepler-9, which has three. The Kepler-11 findings will be published in the Feb. 3 issue of the journal Nature.
• January 10, 2011: NASA's Kepler mission confirmed the discovery of its first rocky planet, named Kepler-10b. Measuring 1.4 times the size of Earth, it is the smallest planet ever discovered outside our solar system. The discovery of this so-called exoplanet is based on more than eight months of data collected by the spacecraft from May 2009 to early January 2010. 104)
Figure 58: Imagined View from Planet Kepler-10b (Artist's depiction), image credit: NASA/Kepler Mission
Legend to Figure 58: Kepler-10b orbits one of the 150,000 stars that the Kepler spacecraft is monitoring, a star that is very similar to our own Sun in temperature, mass and size, but older with an age of over 8 billion years, compared to the 4.5 billion years of our own Sun. It's one of the brighter stars that Kepler is monitoring and about 560 light years from our solar system, which means when the light from this star began its journey toward Earth, European navigators were crossing the Atlantic Ocean for the first time in search of new horizons. Today, we're still exploring and our crow's nest is a space telescope called Kepler. One day, the oceans we cross will be the galaxy itself, but for now, we imagine the worlds we discover by putting all that we've learned from our observations and analyses into the fingers of artists. Kepler-10b must be a scorched world, orbiting at a distance that's more than 20 times closer to its star than Mercury is to our own Sun, with a daytime temperature expected to be more than 2,500 degrees Fahrenheit. The Kepler team has determined that Kepler-10b is a rocky planet, with a surface you could stand on, a mass 4.6 times that of Earth, and a diameter 1.4 times that of Earth. 105)
• Aug. 26, 2010: NASA's Kepler spacecraft has discovered the first confirmed planetary system with more than one planet crossing in front of, or transiting, the same star. — The transit signatures of two distinct planets were seen in the data for the sun-like star designated Kepler-9. The planets were named Kepler-9b and 9c. The discovery incorporates seven months of observations of more than 156,000 stars as part of an ongoing search for Earth-sized planets outside our solar system. The findings will be published in Thursday's issue of the journal Science. 106)
- Kepler-9b is the larger of the two planets, and both have masses similar to but less than Saturn. Kepler-9b lies closest to the star with an orbit of about 19 days, while Kepler-9c has an orbit of about 38 days.
- "This discovery is the first clear detection of significant changes in the intervals from one planetary transit to the next, what we call transit timing variations," said Matthew Holman, a Kepler mission scientist from the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "This is evidence of the gravitational interaction between the two planets as seen by the Kepler spacecraft."
- In addition to the two confirmed giant planets, Kepler scientists also have identified what appears to be a third, much smaller transit signature in the observations of Kepler-9. That signature is consistent with the transits of a super-Earth-sized planet about 1.5 times the radius of Earth in a scorching, near-sun 1.6 day-orbit. Additional observations are required to determine whether this signal is indeed a planet or an astronomical phenomenon that mimics the appearance of a transit.
Figure 59: Worlds on the Edge. This artist's concept illustrates the two Saturn-sized planets discovered by NASA's Kepler mission. The star system is oriented edge-on, as seen by Kepler, such that both planets cross in front, or transit, their star, named Kepler-9. This is the first star system found to have multiple transiting planets (image credit: NASA/Kepler Mission)
• June 16, 2010: NASA's Kepler Mission has released 43 days of science data on more than 156,000 stars. These stars are being monitored for subtle brightness changes as part of an ongoing search for Earth-like planets outside of our solar system. Astronomers will use the new data to determine if orbiting planets are responsible for brightness variations in several hundred stars. These stars make up a full range of temperatures, sizes and ages. Many of them are stable, while others pulsate. Some show starspots, which are similar to sunspots, and a few produce flares that would sterilize their nearest planets. 107)
- "This is the most precise, nearly continuous, longest and largest data set of stellar photometry ever," said Kepler Deputy Principal Investigator David Koch of NASA's Ames Research Center in Moffett Field, Calif. "The results will only get better as the duration of the data set grows with time."
- Kepler will continue conducting science operations until at least November 2012, searching for planets as small as Earth, including those that orbit stars in a warm, habitable zone where liquid water could exist on the surface of the planet. Since transits of planets in the habitable zone of solar-like stars occur about once a year and require three transits for verification, it is expected to take at least three years to locate and verify an Earth-size planet.
• March 4, 2010: One year ago this week, NASA's Kepler Mission soared into the dark night sky, leaving a bright glow in its wake as it began its search for other worlds like Earth. Since the search began, NASA's plucky exoplanet hunter has achieved significant success in its quest to answer the timeless question: "Are we alone in our galaxy?" 108)
• January 4, 2010: NASA's Kepler space telescope, designed to find Earth-size planets in the habitable zone of sun-like stars, has discovered its first five new exoplanets, or planets beyond our solar system. Kepler's high sensitivity to both small and large planets enabled the discovery of the exoplanets, named Kepler 4b, 5b, 6b, 7b and 8b (Figure 60). The discoveries were announced Monday, Jan. 4, by the members of the Kepler science team during a news briefing at the American Astronomical Society meeting in Washington, DC. 109)
- "These observations contribute to our understanding of how planetary systems form and evolve from the gas and dust disks that give rise to both the stars and their planets," said William Borucki of NASA/ARC in Moffett Field, CA. Borucki is the mission's science principal investigator. "The discoveries also show that our science instrument is working well. Indications are that Kepler will meet all its science goals."
- Known as "hot Jupiters" because of their high masses and extreme temperatures, the new exoplanets range in size from similar to Neptune to larger than Jupiter. They have orbits ranging from 3.3 to 4.9 days. Estimated temperatures of the planets range from 1,200 - 1,600 ºC, hotter than molten lava and much too hot for life as we know it. All five of the exoplanets orbit stars hotter and larger than Earth's sun.
Figure 60: The size of Kepler's first five planet discoveries as compared with Jupiter and Earth in our own solar system (image credit: NASA)
• August 6, 2009: NASA's new exoplanet-hunting Kepler space telescope has detected the atmosphere of a known giant gas planet, demonstrating the telescope's extraordinary scientific capabilities. The new data indicate the mission is indeed capable of finding Earth-like planets, if they exist. 110)
- The find is based on a relatively short 10 days of test data collected before the official start of science operations. Kepler was launched March 7, 2009, from Cape Canaveral Air Force Station in Florida. The observation demonstrates the extremely high precision of the measurements made by the telescope, even before its calibration and data analysis software were finished.
- "As NASA's first exoplanets mission, Kepler has made a dramatic entrance on the planet-hunting scene," said Jon Morse, director of the Science Mission Directorate's Astrophysics Division at NASA Headquarters in Washington. "Detecting this planet's atmosphere in just the first 10 days of data is only a taste of things to come. The planet hunt is on!"
- Kepler team members say these new data indicate the mission is indeed capable of finding Earth-like planets, if they exist. Kepler will spend the next three-and-a-half years searching for planets as small as Earth, including those that orbit stars in a warm zone where there could be water. It will do this by looking for periodic dips in the brightness of stars, which occur when orbiting planets transit, or cross in front of, the stars.
- "When the light curves from tens of thousands of stars were shown to the Kepler science team, everyone was awed; no one had ever seen such exquisitely detailed measurements of the light variations of so many different types of stars," said William Borucki, the principal science investigator and lead author of the paper.
- The observations were collected from a planet called HAT-P-7, known to transit a star located about 1,000 light years from Earth. The planet orbits the star in just 2.2 days and is 26 times closer than Earth is to the sun. Its orbit, combined with a mass somewhat larger than the planet Jupiter, classifies this planet as a "hot Jupiter." It is so close to its star, the planet is as hot as the glowing red heating element on a stove.
Figure 61: Comparison of ground-based and space-based light curves for hot exoplanet HAT P7b (image credit: NASA)
- The Kepler measurements show the transit from the previously detected HAT-P-7. However, these new measurements are so precise, they also show a smooth rise and fall of the light between transits caused by the changing phases of the planet, similar to those of our moon. This is a combination of both the light emitted from the planet and the light reflected off the planet. The smooth rise and fall of light is also punctuated by a small drop in light, called an occultation, exactly halfway between each transit. An occultation happens when a planet passes behind a star.
Figure 62: Distributions of mass and orbit size for discovered planets (image credit: NASA)
• May 13, 2009: NASA's Kepler spacecraft has begun its search for other Earth-like worlds. The mission will spend the next 3 1/2 years staring at more than 100,000 stars for telltale signs of planets. Kepler has the unique ability to find planets as small as Earth that orbit sun-like stars at distances where temperatures are right for possible lakes and oceans. 111)
- "Now the fun begins," said William Borucki, Kepler science principal investigator at NASA's Ames Research Center, Moffett Field, CA. "We are all really excited to start sorting through the data and discovering the planets."
- Scientists and engineers have spent the last two months checking out and calibrating the Kepler spacecraft. Data have been collected to characterize the imaging performance as well as the noise level in the measurement electronics. The scientists have constructed the list of targets for the start of the planet search, and this information has been loaded onto the spacecraft.
• April 8, 2009: The image of Figure 63 from NASA's Kepler mission shows the telescope's full field of view — an expansive star-rich patch of sky in the constellations Cygnus and Lyra stretching across 100 square degrees, or the equivalent of two side-by-side dips of the Big Dipper. 112)
- Kepler was designed to hunt for planets like Earth. Of the approximately 4.5 million stars in the region pictured here, more than 100,000 were selected as candidates for Kepler's search. The mission will spend the next 3 1/2 years staring at these target stars, looking for periodic dips in brightness. Such dips occur when planets cross in front of their stars from our point of view in the galaxy, partially blocking the starlight.
- The area in the lower right of the image is brighter because it is closer to the plane of our galaxy and is jam-packed with stars. The area in upper left is farther from the galactic plane and contains fewer stars. The image has been color-coded so that brighter stars appear white, and fainter stars, red. It is a 60 second exposure, taken on April 8, 2009, one day after the spacecraft's dust cover was jettisoned.
- To achieve the level of precision needed to spot planets as small as Earth, Kepler's images are intentionally blurred slightly. This minimizes the number of saturated stars. Saturation, or "blooming," occurs when the brightest stars overload the individual pixels in the detectors, causing the signal to spill out into nearby pixels. These spills can be seen in the image as fine white lines extending above and below some of the brightest stars. Blooming is an expected side effect of Kepler's ultra-sensitive camera. Some of the lightly saturated stars are candidates for planet searches, while those that are heavily saturated are not.
- The grid lines across the picture show how the focal plane is laid out on Kepler's camera — the largest ever launched in space at 95 Mpixels. There are 42 CCDs (Charge-Coupled Devices), paired into square-shaped modules, whose outline can be seen in the image. A thin black line in each module shows adjacent pairs of CCDs. The thicker black lines that cross through the image are from structures holding the modules together, and were purposely oriented to block out the very brightest stars in Kepler's field of view.
- The four black corners of the image show where the fine-guidance sensors reside on the focal plane. These sensors are used to hold the telescope's gaze steady by measuring its position on the sky 10 times/ second, and by feeding this information to the spacecraft's attitude control system.
- Ghost images also appear in the image, which are reflections off the lenses above the CCDs. These expected artifacts were mapped out during ground testing for Kepler, and will not affect science observations because they will be removed as the data are processed.
Figure 63: Full focal plane image (first light for Kepler Photometer), image credit: NASA/Ames/JPL-Caltech