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Kepler Mission - Hunting for Exoplanets

Spacecraft    Launch    Sensor Complement    Ground Segment    Mission Status    References   

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.

The Transit Method of Detecting Extrasolar Planets: 13) 14)

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.

Spacecraft dimensions

The overall size is about 2.7 m in diameter and 4.7 m high.

Spacecraft mass

Launch mass of 1052.4 kg, consisting of 562.7 kg for the spacecraft, 478 kg for the photometer, and 11.7 kg of hydrazine propellant


Power is provided by four non-coplanar panels with a total area of 10.2 m2 of solar collecting surface area. Combined, the 2860 individual solar cells can produce over 1,100 W.. Power storage is provided by a 20 Amp-hour rechargeable lithium-ion battery.

Fine pointing

The spacecraft is three-access stabilized to better than 9 milliarcseconds, 3σ over 15 minutes using fine-guidance sensors located on the instrument focal plane.


X-band is used for uplink commanding and realtime engineering data downlinking. Ka-band is used to downlink the stored science and engineering data.

Data storage capability

About 60 days

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)

Spaceborne Photometer

0.95 m aperture

Primary mirror

1.4 m diameter, 85% light weighted


95 Mpixels (42 CCDs with 2200 x 1024 pixels)


430-890 nm FWHM (Full Width Half Maximum)

Dynamic range

9th to 15th magnitude stars

Fine guidance sensors

4 CCDs located on science FPA (Focal Plane Array)

Attitude stability

better than 9 marcsec, 3σ over 15 minutes

Science data storage

about 2 months

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)


Figure 8: Illustration of the Photometer in the Kepler telescope shell (image credit: NASA, Kepler Team, Ref. 19)

Photometer instrument:

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)

Ground Segment:

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: (Kepler Observatory was launched on 7 March 2009 and ended its mission on 15 November 2018)

• October 29, 2020: Since astronomers confirmed the presence of planets beyond our solar system, called exoplanets, humanity has wondered how many could harbor life. Now, we’re one step closer to finding an answer. According to new research using data from NASA’s retired planet-hunting mission, the Kepler space telescope, about half the stars similar in temperature to our Sun could have a rocky planet capable of supporting liquid water on its surface. 25)

- Our galaxy holds an estimated 300 million of these potentially habitable worlds, based on results in a study released today and to be published in The Astronomical Journal. Some of these exoplanets could even be our interstellar neighbors, with four potentially within 30 light-years of our Sun and the closest likely to be about 20 light-years from us.

- This research helps us understand the potential for these planets to have the elements to support life. This is an essential part of astrobiology, the study of life’s origins and future in our universe.

- The study is authored by NASA scientists who worked on the Kepler mission alongside collaborators from around the world. NASA retired the space telescope in 2018 after it ran out of fuel. Nine years of the telescope’s observations revealed that there are billions of planets in our galaxy – more planets than stars.

- "Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable," said the lead author Steve Bryson, a researcher at NASA's Ames Research Center in California's Silicon Valley. "Though this result is far from a final value, and water on a planet's surface is only one of many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision."

- For the purposes of calculating this occurrence rate, the team looked at exoplanets between a radius of 0.5 and 1.5 times that of Earth's, narrowing in on planets that are most likely rocky. They also focused on stars similar to our Sun in age and temperature, plus or minus up to 1,500 degrees Fahrenheit.

- That's a wide range of different stars, each with its own particular properties impacting whether the rocky planets in its orbit are capable of supporting liquid water. These complexities are partly why it is so difficult to calculate how many potentially habitable planets are out there, especially when even our most powerful telescopes can just barely detect these small planets. That's why the research team took a new approach.

Figure 15: This illustration depicts one possible appearance of the planet Kepler-452b, the first near-Earth-size world to be found in the habitable zone of a star similar to our Sun (image credits: NASA Ames/JPL-Caltech/T. Pyle)

Rethinking How to Identify Habitability

- This new finding is a significant step forward in Kepler's original mission to understand how many potentially habitable worlds exist in our galaxy. Previous estimates of the frequency, also known as the occurrence rate, of such planets ignored the relationship between the star's temperature and the kinds of light given off by the star and absorbed by the planet.

- The new analysis accounts for these relationships, and provides a more complete understanding of whether or not a given planet might be capable of supporting liquid water, and potentially life. That approach is made possible by combining Kepler's final dataset of planetary signals with data about each star's energy output from an extensive trove of data from the European Space Agency's Gaia mission.

- "We always knew defining habitability simply in terms of a planet's physical distance from a star, so that it's not too hot or cold, left us making a lot of assumptions," said Ravi Kopparapu, an author on the paper and a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Gaia's data on stars allowed us to look at these planets and their stars in an entirely new way."

- Gaia provided information about the amount of energy that falls on a planet from its host star based on a star's flux, or the total amount of energy that is emitted in a certain area over a certain time. This allowed the researchers to approach their analysis in a way that acknowledged the diversity of the stars and solar systems in our galaxy.

- "Not every star is alike," said Kopparapu. "And neither is every planet."

- Though the exact effect is still being researched, a planet's atmosphere figures into how much light is needed to allow liquid water on a planet's surface as well. Using a conservative estimate of the atmosphere's effect, the researchers estimated an occurrence rate of about 50% — that is, about half of Sun-like stars have rocky planets capable of hosting liquid water on their surfaces. An alternative optimistic definition of the habitable zone estimates about 75%.

Kepler's Legacy Charts Future Research

- This result builds upon a long legacy of work of analyzing Kepler data to obtain an occurrence rate and sets the stage for future exoplanet observations informed by how common we now expect these rocky, potentially habitable worlds to be. Future research will continue to refine the rate, informing the likelihood of finding these kinds of planets and feeding into plans for the next stages of exoplanet research, including future telescopes.

- ”Knowing how common different kinds of planets are is extremely valuable for the design of upcoming exoplanet-finding missions," said co-author Michelle Kunimoto, who worked on this paper after finishing her doctorate on exoplanet occurrence rates at the University of British Columbia, and recently joined the TESS (Transiting Exoplanet Survey Satellite) team at the Massachusetts Institute of Technology in Cambridge, Massachusetts. "Surveys aimed at small, potentially habitable planets around Sun-like stars will depend on results like these to maximize their chance of success."


Figure 16: An illustration representing the legacy of NASA's Kepler space telescope. After nine years in deep space collecting data that revealed our night sky to be filled with billions of hidden planets – more planets even than stars – NASA’s Kepler space telescope ran out of fuel needed for further science operations in 2018 (image credits: NASA/Ames Research Center/W. Stenzel/D. Rutter)

- After revealing more than 2,800 confirmed planets outside our solar system, the data collected by the Kepler space telescope continues to yield important new discoveries about our place in the universe. Though Kepler's field of view covered only 0.25% of the sky, the area that would be covered by your hand if you held it up at arm's length towards the sky, its data has allowed scientists to extrapolate what the mission's data means for the rest of the galaxy. That work continues with TESS, NASA's current planet hunting telescope.

- "To me, this result is an example of how much we've been able to discover just with that small glimpse beyond our solar system," said Bryson. "What we see is that our galaxy is a fascinating one, with fascinating worlds, and some that may not be too different from our own."


Figure 17: This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone (image credit: NASA Ames/JPL-Caltech/T. Pyle)

• April 30, 2020: The Sun is an ever-changing star: at times, numerous dark sunspots cover its visible surface; at others, the surface is completely "empty". However, by cosmic standards the Sun is extraordinarily monotonous. This is the result of a new study presented by researchers under the leadership of the Max Planck Institute for Solar System Research (MPS) in the upcoming issue of Science. For the first time, the scientists compared the Sun with hundreds of other stars with similar rotation periods and other fundamental properties. Most of them displayed much stronger variations. This raises the question of whether the Sun’s feebleness is a basic trait or whether our star has merely been going through an unusually quiet phase for several millennia. 26) 27)

- The extent to which solar activity (and thus the number of sunspots and the solar brightness) varies can be reconstructed using various methods - at least for a certain period of time. Since 1610, for example, there have been reliable records of sunspots covering the Sun; the distribution of radioactive varieties of carbon and beryllium in tree rings and ice cores allows us to draw conclusions about the level of solar activity over the past 9000 years. For this period of time, scientists find regularly recurring fluctuations of comparable strength as during recent decades. "However, compared to the entire lifespan of the Sun, 9000 years is like the blink of an eye”, says MPS scientist Dr. Timo Reinhold, first author of the new study. After all, our star is almost 4.6 billion years old. "It is conceivable that the Sun has been going through a quiet phase for thousands of years and that we therefore have a distorted picture of our star," he adds.

- Since there is no way of finding out how active the Sun was in primeval times, scientists can only resort to the stars: Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity.

Figure 18: The lightcurve of the Sun. The dark spots on the sun gradually disappear from view, but continue to develop and reappear again after one rotation period. This causes the brightness of the sun to fluctuate (video credit: MPS)

Figure 19: The lightcurve of a star. The fluctuations of a typical sunlike star are much stronger than those of the Sun (video credit: MPS)

- To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "The speed at which a star rotates around its own axis is a crucial variable", explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication. A star’s rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki. The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are - and thus also how brightly the Sun shines.

- A comprehensive catalog containing the rotation periods of thousands of stars has been available only for the last few years. It is based on measurement data from NASA’s Kepler Space Telescope, which recorded the brightness fluctuations of approximately 150,000 main sequence stars (i.e. those that are in the middle of their lifetimes) from 2009 to 2013. The researchers scoured this huge sample and selected those stars that rotate once around their own axis within 20 to 30 days. The Sun needs about 24.5 days for this. The researchers were able to further narrow down this sample by using data from the European Gaia Space Telescope. In the end, 369 stars remained, which also resemble the Sun in other fundamental properties.

- The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong. "We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities".

- However, it is not possible to determine the rotation period of all the stars observed by the Kepler telescope. To do this, scientists have to find certain periodically re-appearing dips in the star’s lightcurve. These dips can be traced back to starspots that darken the stellar surface, rotate out of the telescope's field of view and then reappear after a fixed period of time. "For many stars, such periodic darkenings cannot be detected; they are lost in the noise of the measured data and in overlying brightness fluctuations," explains Reinhold. Viewed through the Kepler telescope, even the Sun would not reveal its rotation period.


Figure 20: Not very active: comparison of the brightness variations of the Sun with those of a typical Sun-like star (image credit: MPS /

- The researchers therefore also studied more than 2500 Sun-like stars with unknown rotation periods. Their brightness fluctuated much less than that of the other group.

- These results allow two interpretations. There could be a still unexplained fundamental difference between stars with known and unknown rotation period. "It is just as conceivable that stars with known and Sun-like rotation periods show us the fundamental fluctuations in activity the Sun is capable of," says Shapiro. This would mean that our star has been unusually feeble over the past 9000 years and that on very large time scales phases with much greater fluctuations are also possible.

- There is, however, no cause for concern. For the foreseeable future, there is no indication of such solar "hyperactivity". On the contrary: For the last decade, the Sun has been showing itself to be rather weakly active, even by its own low standards. Predictions of activity for the next eleven years indicate that this will not change soon.

• April 15, 2020: A team of transatlantic scientists, using reanalyzed data from NASA’s Kepler space telescope, has discovered an Earth-size exoplanet orbiting in its star's habitable zone, the area around a star where a rocky planet could support liquid water. 28)

- Scientists discovered this planet, called Kepler-1649c, when looking through old observations from Kepler, which the agency retired in 2018. While previous searches with a computer algorithm misidentified it, researchers reviewing Kepler data took a second look at the signature and recognized it as a planet. Out of all the exoplanets found by Kepler, this distant world – located 300 light-years from Earth – is most similar to Earth in size and estimated temperature.


Figure 21: An illustration of Kepler-1649c orbiting around its host red dwarf star. This newly discovered exoplanet is in its star’s habitable zone and is the closest to Earth in size and temperature found yet in Kepler's data (image credit: NASA/Ames Research Center/Daniel Rutter)

- This newly revealed world is only 1.06 times larger than our own planet. Also, the amount of starlight it receives from its host star is 75% of the amount of light Earth receives from our Sun – meaning the exoplanet's temperature may be similar to our planet’s, as well. But unlike Earth, it orbits a red dwarf. Though none have been observed in this system, this type of star is known for stellar flare-ups that may make a planet's environment challenging for any potential life.

- "This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington. “The data gathered by missions like Kepler and our Transiting Exoplanet Survey Satellite (TESS) will continue to yield amazing discoveries as the science community refines its abilities to look for promising planets year after year."

- There is still much that is unknown about Kepler-1649c, including its atmosphere, which could affect the planet's temperature. Current calculations of the planet's size have significant margins of error, as do all values in astronomy when studying objects so far away. Rocky planets orbiting red dwarfs are of particular astrobiological interest. However, astrobiologists will need much more information about this planet in order to gage whether it is promising for life as we know it. But based on what is known, Kepler-1649c is especially intriguing for scientists looking for worlds with potentially habitable conditions.

- There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c. Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d. But there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system.

- "Out of all the mislabeled planets we've recovered, this one's particularly exciting - not just because it's in the habitable zone and Earth-size, but because of how it might interact with this neighboring planet," said Andrew Vanderburg, a researcher at the University of Texas at Austin and first author on the paper released today in The Astrophysical Journal Letters. "If we hadn't looked over the algorithm's work by hand, we would have missed it." 29)

- Kepler-1649c orbits its small red dwarf star so closely that a year on Kepler-1649c is equivalent to only 19.5 Earth days. The system has another rocky planet of about the same size, but it orbits the star at about half the distance of Kepler-1649c, similar to how Venus orbits our Sun at about half the distance that Earth does. Red dwarf stars are among the most common in the galaxy, meaning planets like this one could be more common than we previously thought.

Looking for False Positives

- Previously, scientists on the Kepler mission developed an algorithm called Robovetter to help sort through the massive amounts of data produced by the Kepler spacecraft, managed by NASA's Ames Research Center in California's Silicon Valley. Kepler searched for planets using the transit method, staring at stars, looking for dips in brightness as planets passed in front of their host stars.

- Most of the time, those dips come from phenomena other than planets – ranging from natural changes in a star's brightness to other cosmic objects passing by – making it look like a planet is there when it's not. Robovetter's job was to distinguish the 12% of dips that were real planets. Those signatures Robovetter determined to be from other sources were labeled "false positives," the term for a test result mistakenly classified as positive.

- With an enormous number of tricky signals, astronomers knew the algorithm would make mistakes and would need to be double-checked – a perfect job for the Kepler False Positive Working Group. That team reviews Robovetter's work, going through all false positives to ensure they are truly errors and not exoplanets, ensuring fewer potential discoveries are overlooked. As it turns out, Robovetter had mislabeled Kepler-1649c.

- Even as scientists work to further automate analysis processes to get the most science as possible out of any given dataset, this discovery shows the value of double-checking automated work. Even six years after Kepler stopped collecting data from the original Kepler field – a patch of sky it stared at from 2009 to 2013, before going on to study many more regions – this rigorous analysis uncovered one of the most unique Earth-analogs discovered yet.

A Possible Third Planet

- Kepler-1649c not only is one of the best matches to Earth in terms of size and energy received from its star, but it provides an entirely new look at its home system. For every nine times the outer planet in the system orbits the host star, the inner planet orbits almost exactly four times. The fact that their orbits match up in such a stable ratio indicates the system itself is extremely stable, and likely to survive for a long time.

- Nearly perfect period ratios are often caused by a phenomenon called orbital resonance, but a nine-to-four ratio is relatively unique among planetary systems. Usually resonances take the form of ratios such as two-to-one or three-to-two. Though unconfirmed, the rarity of this ratio could hint to the presence of a middle planet with which both the inner and outer planets revolve in synchronicity, creating a pair of three-to-two resonances.

- The team looked for evidence of such a mystery third planet, with no results. However, that could be because the planet is too small to see or at an orbital tilt that makes it impossible to find using Kepler's transit method.

- Either way, this system provides yet another example of an Earth-size planet in the habitable zone of a red dwarf star. These small and dim stars require planets to orbit extremely close to be within that zone – not too warm and not too cold – for life as we know it to potentially exist. Though this single example is only one among many, there is increasing evidence that such planets are common around red dwarfs.

- "The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-size exoplanets are common around these kinds of stars," said Vanderburg. "With red dwarfs almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them, the chance one of them isn't too different than our Earth looks a bit brighter."

- Missions such as Kepler and TESS help contribute to the field of astrobiology, the interdisciplinary research into understanding how the variables and environmental conditions of distant worlds could harbor life as we know it, or whatever other form that life could take.

• February 28, 2020: UBC (University of British Columbia) astronomy student Michelle Kunimoto has discovered 17 new planets, including a potentially habitable, Earth-sized world, by combing through data gathered by NASA’s Kepler mission. 30)

- Over its original four-year mission, the Kepler satellite looked for planets, especially those that lie in the “Habitable Zones” of their stars, where liquid water could exist on a rocky planet’s surface.

- The new findings, published in The Astronomical Journal, include one such, particularly rare planet. Officially named KIC-7340288 b, the planet discovered by Kunimoto is just 1 ½ times the size of Earth – small enough to be considered rocky, instead of gaseous like the giant planets of the Solar System – and in the habitable zone of its star. 31)


Figure 22: Michelle Kunimoto has discovered 17 new planets (photo credit: UBC)

- “This planet is about a thousand light years away, so we’re not getting there anytime soon!” said Kunimoto, a PhD candidate in the department of physics and astronomy. “But this is a really exciting find, since there have only been 15 small, confirmed planets in the Habitable Zone found in Kepler data so far.”

- The planet has a year that is 142 ½ days long, orbiting its star at 0.444 Astronomical Units (AU, the distance between Earth and our Sun) – just bigger than Mercury’s orbit in our Solar System, and gets about a third of the light Earth gets from the Sun.


Figure 23: Sizes of the 17 new planet candidates, compared to Mars, Earth, and Neptune. The planet in green is KIC-7340288 b, a rare rocky planet in the Habitable Zone (image credit: Michelle Kunimoto)

- Of the other 16 new planets discovered, the smallest is only two-thirds the size of Earth - one of the smallest planets to be found with Kepler so far. The rest range in size up to eight times the size of Earth.

- Kunimoto is no stranger to discovering planets: she previously discovered four during her undergraduate degree at UBC. Now working on her PhD at UBC, she used what is known as the “transit method” to look for the planets among the roughly 200,000 stars observed by the Kepler mission.

- “Every time a planet passes in front of a star, it blocks a portion of that star’s light and causes a temporary decrease in the star’s brightness,” Kunimoto said. “By finding these dips, known as transits, you can start to piece together information about the planet, such as its size and how long it takes to orbit.”

- Kunimoto also collaborated with UBC alumnus Henry Ngo to obtain razor-sharp follow-up images of some of her planet-hosting stars with the Near InfraRed Imager and Spectrometer (NIRI) on the Gemini North 8-meter Telescope in Hawaii.

- “I took images of the stars as if from space, using adaptive optics,” she said. “I was able to tell if there was a star nearby that could have affected Kepler’s measurements, such as being the cause of the dip itself.”

- In addition to the new planets, Kunimoto was able to observe thousands of known Kepler planets using the transit-method, and will be reanalyzing the exoplanet census as a whole.

- “We’ll be estimating how many planets are expected for stars with different temperatures,” said Kunimoto’s PhD supervisor and UBC professor Jaymie Matthews. “A particularly important result will be finding a terrestrial Habitable Zone planet occurrence rate. How many Earth-like planets are there? Stay tuned.”

• July 30, 2019: Exoplanets revolving around distant stars are coming quickly into focus with advanced technology like the Kepler space telescope. Gaining a full understanding of those systems is difficult, because the initial positions and velocities of the exoplanets are unknown. Determining whether the system dynamics are quasi-periodic or chaotic is cumbersome, expensive and computationally demanding. 32) 33)

- In this week's Chaos, from AIP Publishing, Tamás Kovács delivers an alternative method for stability analysis of exoplanetary bodies using only the observed time series data to deduce dynamical measurements and quantify the unpredictability of exoplanet systems.

- "If we don't know the governing equations of the motion of a system, and we only have the time series — what we measure with the telescope — then we want to transform that time series into a complex network. In this case, it is called a recurrence network," Kovács said. "This network holds all of the dynamical features of the underlying system we want to analyze."

- The paper draws on the work of physicist Floris Takens, who proposed in 1981 that the dynamics of a system could be reconstructed using a series of observations about the state of the system. With Takens' embedding theorem as a starting point, Kovács uses time delay embedding to reconstruct a high-dimensional trajectory and then identify recurrence points, where bodies in the phase space are close to each other.

- "Those special points will be the vertices and the edges of the complex network," Kovács said. "Once you have the network, you can reprogram this network to be able to apply measures like transitivity, average path length or others unique to that network."

- Kovács tests the reliability of the method using a known system as a model, the three-body system of Saturn, Jupiter and the sun, and then applies it to the Kepler 36b and 36c system. His Kepler system results agree with what is known.

- "Earlier studies pointed out that Kepler 36b and 36c is a very special system, because from the direct simulation and the numerical integrations, we see the system is at the edge of the chaos," Kovács said. "Sometimes, it shows regular dynamics, and at other times, it seems to be chaotic."

- The author plans to next apply his methods to systems with more than three bodies, testing its scalability and exploring its ability to handle longer time series and sharper datasets.

• April 16, 2019: Astronomers have discovered a third planet in the Kepler-47 system, securing the system's title as the most interesting of the binary-star worlds. Using data from NASA's Kepler space telescope, a team of researchers, led by astronomers at SDSU (San Diego State University), detected the new Neptune-to-Saturn-size planet orbiting between two previously known planets. 34)


Figure 24: This is an artistic rendition of the Kepler-47 circumbinary planet system. The three planets with the large middle planet being the newly discovered Kepler47d (image credit: NASA/JPL, Caltech, T. Pyle)

- With its three planets orbiting two suns, Kepler-47 is the only known multi-planet circumbinary system. Circumbinary planets are those that orbit two stars.

- The planets in the Kepler-47 system were detected via the "transit method." If the orbital plane of the planet is aligned edge-on as seen from Earth, the planet can pass in front of the host stars, leading to a measurable decrease in the observed brightness. The new planet, dubbed Kepler-47d, was not detected earlier due to weak transit signals.

- As is common with circumbinary planets, the alignment of the orbital planes of the planets change with time. In this case, the middle planet's orbit has become more aligned, leading to a stronger transit signal. The transit depth went from undetectable at the beginning of the Kepler Mission to the deepest of the three planets over the span of just four years.

- The SDSU researchers were surprised by both the size and location of the new planet. Kepler-47d is the largest of the three planets in the Kepler-47 system.

- "We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure," said SDSU astronomer Jerome Orosz, the paper's lead author. "With an additional transit, the planet's orbital period could be determined, and we were then able to uncover more transits that were hidden in the noise in the earlier data."

- William Welsh, SDSU astronomer and the study's co-author, said he and Orosz expected any additional planets in the Kepler-47 system to be orbiting exterior to the previously known planets. "We certainly didn't expect it to be the largest planet in the system. This was almost shocking," said Welsh. Their research was recently published in the Astronomical Journal. 35)

- With the discovery of the new planet, a much better understanding of the system is possible. For example, researchers now know the planets in this circumbinary system are very low density — less than that of Saturn, the Solar System planet with the lowest density.

- While a low density is not that unusual for the sizzling hot-Jupiter type exoplanets, it is rare for mild-temperature planets. Kepler-47d's equilibrium temperature is roughly 50ºF (10ºC), while Kepler-47c is 26ºF (32ºC). The innermost planet, which is the smallest circumbinary planet known, is a much hotter 336ºF (169ºC).

- The inner, middle, and outer planets are 3.1, 7.0, and 4.7 times the size of the Earth, and take 49, 87, and 303 days, respectively, to orbit around their suns. The stars themselves orbit each other in only 7.45 days; one star is similar to the Sun, while the other has a third of the mass of the Sun. The entire system is compact and would fit inside the orbit of the Earth. It is approximately 3340 light-years away in the direction of the constellation Cygnus.

- "This work builds on one of the Kepler's most interesting discoveries: that systems of closely-packed, low-density planets are extremely common in our galaxy," said University of California, Santa Cruz astronomer Jonathan Fortney, who was not part of the study. "Kepler47 shows that whatever process forms these planets — an outcome that did not happen in our solar system -is common to single-star and circumbinary planetary systems."

- This work was supported in part by grants from NASA and the National Science Foundation.

• March 26, 2019: Astronomers at The University of Texas at Austin, in partnership with Google, have used artificial intelligence (AI) to uncover two more hidden planets in the Kepler space telescope archive. The technique shows promise for identifying many additional planets that traditional methods could not catch. - The planets discovered this time were from Kepler's extended mission, called K2. 36)

- To find them, the team, led by an undergraduate at UT Austin, Anne Dattilo, created an algorithm that sifts through the data taken by Kepler to ferret out signals that were missed by traditional planet-hunting methods. I n the long term, the process should help astronomers find many more missed planets hiding in Kepler data. The discoveries have been accepted for publication in an upcoming issue of The Astronomical Journal. 37)

- Other team members include NASA Sagan fellow at UT Austin Andrew Vanderburg and Google engineer Christopher Shallue. In 2017, Vanderburg and Shallue first used AI to uncover a planet around a Kepler star — one already known to harbor seven planets. The discovery made that solar system the only one known to have as many planets as our own.

- Dattilo explained that this project necessitated a new algorithm, as data taken during Kepler's extended mission K2 differs significantly from that collected during the spacecraft's original mission.

- "K2 data is more challenging to work with because the spacecraft is moving around all the time," Vanderburg explained. This change came about after a mechanical failure. While mission planners found a workaround, the spacecraft was left with a wobble that AI had to take into account.

- The Kepler and K2 missions have already discovered thousands of planets around other stars, with an equal number of candidates awaiting confirmation. So why do astronomers need to use AI to search the Kepler archive for more?

- "AI will help us search the data set uniformly," Vanderburg said. "Even if every star had an Earth-sized planet around it, when we look with Kepler, we won't find all of them. That's just because some of the data's too noisy, or sometimes the planets are just not aligned right. So, we have to correct for the ones we missed. We know there are a lot of planets out there that we don't see for those reasons.

- "If we want to know how many planets there are in total, we have to know how many planets we've found, but we also have to know how many planets we missed. That's where this comes in," he explained.

- The two planets Dattilo's team found "are both very typical of planets found in K2," she said. "They're really close in to their host star, they have short orbital periods, and they're hot. They are slightly larger than Earth."

- Of the two planets, one is called K2-293b and orbits a star 1,300 light-years away in the constellation Aquarius. The other, K2-294b, orbits a star 1,230 light-years away, also located in Aquarius.

- Once the team used their algorithm to find these planets, they followed up by studying the host stars using ground-based telescopes to confirm that the planets are real. These observations were done with the 1.5-meter telescope at the Smithsonian Institution's Whipple Observatory in Arizona and the Gillett Telescope at Gemini Observatory in Hawaii.

- The future of the AI concept for finding planets hidden in data sets looks bright. The current algorithm can be used to probe the entire K2 data set, Dattilo said — approximately 300,000 stars. She also believes the method is applicable to Kepler's successor planet-hunting mission, TESS, which launched in April 2018. Kepler's mission ended later that year.

- Dattilo plans to continue her work using AI for planet hunting when she enters graduate school in the fall.

• March 5, 2019: An international team of astronomers, led by University of Hawai'i graduate student Ashley Chontos, announced the confirmation of the first exoplanet candidate identified by NASA's Kepler Mission. The result was presented today at the fifth Kepler/K2 Science Conference held in Glendale, CA. 38) 39) 40)

- Launched 10 years ago (on 7 March 2009), the Kepler Space Telescope has discovered thousands of exoplanets using the transit method - small dips in a star's brightness as planets cross in front of the star. Because other phenomena can mimic transits, Kepler data reveal planet candidates, but further analysis is required to confirm them as genuine planets.


Figure 25: Artist's concept of a Kepler-1658-like system. Sound waves propagating through the stellar interior were used to characterize the star and the planet. Kepler-1658b, orbiting with a period of just 3.8 days, was the first exoplanet candidate discovered by Kepler nearly 10 years ago (image credit: Gabriel Perez Diaz/Instituto de Astrofísica de Canarias)

- Despite being the very first planet candidate discovered by NASA's Kepler Space Telescope, the object now known as Kepler-1658 b had a rocky road to confirmation. The initial estimate of the size of the planet's host star was incorrect, so the sizes of both the star and Kepler-1658 b were vastly underestimated. It was later set aside as a false positive when the numbers didn't quite make sense for the effects seen on its star for a body of that size. Fortuitously, Chontos' first year graduate research project, which focused on re-analyzing Kepler host stars, happened at just the right time.

- "Our new analysis, which uses stellar sound waves observed in the Kepler data to characterize the host star, demonstrated that the star is in fact three times larger than previously thought. This in turn means that the planet is three times larger, revealing that Kepler-1658 b is actually a hot Jupiter-like planet," said Ashley Chontos. With this refined analysis, everything pointed to the object truly being a planet, but confirmation from new observations was still needed.

- "We alerted Dave Latham (a senior astronomer at the Smithsonian Astrophysical Observatory, and co-author on the paper) and his team collected the necessary spectroscopic data to unambiguously show that Kepler-1658 b is a planet," said Dan Huber, co-author and astronomer at the University of Hawai'i. "As one of the pioneers of exoplanet science and a key figure behind the Kepler mission, it was particularly fitting to have Dave be part of this confirmation."

- Kepler-1658 is 50% more massive and three times larger than the Sun. The newly confirmed planet orbits at a distance of only twice the star's diameter, making it one of the closest-in planets around a more evolved star - one that resembles a future version of our Sun. Standing on the planet, the star would appear 60 times larger in diameter than the Sun as seen from Earth.

- Planets orbiting evolved stars similar to Kepler-1658 are rare, and the reason for this absence is poorly understood. The extreme nature of the Kepler-1658 system allows astronomers to place new constraints on the complex physical interactions that can cause planets to spiral into their host stars. The insights gained from Kepler-1658b suggest that this process happens slower than previously thought, and therefore may not be the primary reason for the lack of planets around more evolved stars.

- "Kepler-1658 is a perfect example of why a better understanding of host stars of exoplanets is so important." said Chontos. "It also tells us that there are many treasures left to be found in the Kepler data."

• November 16, 2018: The Kepler space telescope has had a profound impact on our understanding of the number of worlds that exist beyond our solar system. Through its survey, we’ve discovered there are more planets than stars in our galaxy. As a farewell to the spacecraft, we asked some of people closest to Kepler to reflect on what Kepler has meant to them and its finding of “more planets than stars.”

Figure 26: Reflections from NASA's Kepler Mission (video credit: NASA's Ames Research Center)

• January 7, 2018: Using data from NASA's Kepler space telescope, citizen scientists have discovered a planet roughly twice the size of Earth located within its star's habitable zone, the range of orbital distances where liquid water may exist on the planet's surface. The new world, known as K2-288Bb, could be rocky or could be a gas-rich planet similar to Neptune. Its size is rare among exoplanets - planets beyond our solar system. 41)


Figure 27: The newfound planet K2-288Bb, illustrated here, is slightly smaller than Neptune. Located about 226 light-years away, it orbits the fainter member of a pair of cool M-type stars every 31.3 days (image credit: NASA's Goddard Space Flight Center/Francis Reddy)

- "It's a very exciting discovery due to how it was found, its temperate orbit and because planets of this size seem to be relatively uncommon," said Adina Feinstein, a University of Chicago graduate student who discussed the discovery on Monday, Jan. 7, at the 233rd meeting of the American Astronomical Society in Seattle. She is also the lead author of a paper describing the new planet accepted for publication by The Astronomical Journal.

- Located 226 light-years away in the constellation Taurus, the planet lies in a stellar system known as K2-288, which contains a pair of dim, cool M-type stars separated by about 8.2 billion kilometers - roughly six times the distance between Saturn and the Sun. The brighter star is about half as massive and large as the Sun, while its companion is about one-third the Sun's mass and size. The new planet, K2-288Bb, orbits the smaller, dimmer star every 31.3 days.

- In 2017, Feinstein and Makennah Bristow, an undergraduate student at the University of North Carolina Asheville, worked as interns with Joshua Schlieder, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. They searched Kepler data for evidence of transits, the regular dimming of a star when an orbiting planet moves across the star's face.

- Examining data from the fourth observing campaign of Kepler's K2 mission, the team noticed two likely planetary transits in the system. But scientists require a third transit before claiming the discovery of a candidate planet, and there wasn't a third signal in the observations they reviewed.

- As it turned out, though, the team wasn't actually analyzing all of the data. - In Kepler's K2 mode, which ran from 2014 to 2018, the spacecraft repositioned itself to point at a new patch of sky at the start of each three-month observing campaign. Astronomers were initially concerned that this repositioning would cause systematic errors in measurements.

- "Re-orienting Kepler relative to the Sun caused miniscule changes in the shape of the telescope and the temperature of the electronics, which inevitably affected Kepler's sensitive measurements in the first days of each campaign," said co-author Geert Barentsen, an astrophysicist at NASA's Ames Research Center in California's Silicon Valley and the director of the guest observer office for the Kepler and K2 missions.

- To deal with this, early versions of the software that was used to prepare the data for planet-finding analysis simply ignored the first few days of observations - and that's where the third transit was hiding.

- As scientists learned how to correct for these systematic errors, this trimming step was eliminated - but the early K2 data Barstow studied had been clipped.

- "We eventually re-ran all data from the early campaigns through the modified software and then re-ran the planet search to get a list of candidates, but these candidates were never fully visually inspected," explained Schlieder, a co-author of the paper. "Inspecting, or vetting, transits with the human eye is crucial because noise and other astrophysical events can mimic transits."

- Instead, the re-processed data were posted directly to Exoplanet Explorers, a project where the public searches Kepler's K2 observations to locate new transiting planets. In May 2017, volunteers noticed the third transit and began an excited discussion about what was then thought to be an Earth-sized candidate in the system, which caught the attention of Feinstein and her colleagues.

- "That's how we missed it - and it took the keen eyes of citizen scientists to make this extremely valuable find and point us to it," Feinstein said.

- The team began follow-up observations using NASA's Spitzer Space Telescope, the Keck II telescope at the W. M. Keck Observatory and NASA's Infrared Telescope Facility (the latter two in Hawaii), and also examined data from ESA's (the European Space Agency's) Gaia mission.

- Estimated to be about 1.9 times Earth's size, K2-288Bb is half the size of Neptune. This places the planet within a recently discovered category called the Fulton gap, or radius gap. Among planets that orbit close to their stars, there's a curious dearth of worlds between about 1.5 and two times Earth's size. This is likely the result of intense starlight breaking up atmospheric molecules and eroding away the atmospheres of some planets over time, leaving behind two populations. Since K2-288Bb's radius places it in this gap, it may provide a case study of planetary evolution within this size range.

- On Oct. 30, 2018, Kepler ran out of fuel and ended its mission after nine years, during which it discovered 2,600 confirmed planets around other stars - the bulk of those now known - along with thousands of additional candidates astronomers are working to confirm. And while NASA's Transiting Exoplanet Survey Satellite is the newest space-based planet hunter, this new finding shows that more discoveries await scientists in Kepler data.

• November 30, 2018: An international research team including The Australian National University (ANU) has used the Kepler space telescope in coordination with ground-based telescopes to witness the first moments of a star dying in unprecedented detail. 42)

- The astronomers witnessed the star dying a long time ago in a galaxy far, far away, as part of a project that aims to solve the mystery of how stars explode.

- Dr. Brad Tucker, one of the lead researchers of the survey, said about 170 million years later on 4 February 2018 the array of high-powered telescopes detected the light emanating from the exploding star, otherwise known as a supernova called SN 2018oh.

- "Kepler—in its final days before running out of fuel and being retired—observed the minute changes in brightness of the star's explosion from its very beginnings, while the ground-based telescopes detected changes in color and the atomic make-up of this dying star," said Dr. Tucker from the ANU Research School of Astronomy and Astrophysics. "With the combined data from these telescopes, astronomers achieved what they had hoped for—an unprecedented observation of the onset of a star's death."

- SN 2018oh is an example of a Type Ia supernova—the kind that astronomers use to measure the expansion of the Universe and probe the nature of dark energy. "Prior to Kepler, it was nearly impossible to study the early stages of a star explosion," Dr. Tucker said.

- A typical Type Ia supernova brightens over the course of three weeks before gradually fading away, but this supernova brightened rapidly a few days after the initial explosion—about three times faster than a typical supernova at this time period.

- The Dark Energy Camera at Cerro Tololo Inter-American Observatory in Chile and the Panoramic Survey Telescope and Rapid Response System at Haleakala Observatory in Hawaii revealed this supernova gleaming blue during this intense period of intensity, an indication of extremely high temperatures—billions of degrees hot.

- Dr. Tucker said some theoretical models propose that an exploding white dwarf—a star that has exhausted its nuclear fuel—hits a neighboring star to cause a supernova, which appears to be the cause of SN 2018oh.

- "With this latest result, we now know a range of star systems cause these important explosions—those used by ANU Vice-Chancellor and astronomer Brian Schmidt to show the Universe was growing at an accelerating rate," he said.

- "The now retired Kepler Space telescope changed our view of the Universe—showing just how common planets around other stars are. It has also now revolutionized what we know about how stars end their lives in brilliant explosions."

- Dr. Tucker said finding out the frequency and distribution of this kind of Type Ia supernova would help to refine the models used in cosmology to estimate the rate of expansion of the Universe.

Three papers by 130 scientists on this study will be published in the Astrophysical Journal Letters and the Astrophysical Journal. 43) 44) 45)


Figure 28: The supernova—known as SN 2018oh—is located in a spiral galaxy called UGC 4780 in the constellation Cancer at a distance of more than 170 million light years (image credit: NASA)

• On the evening of Thursday, 15 November 2018, NASA's Kepler space telescope received its final set of commands to disconnect communications with Earth. The "goodnight" commands finalize the spacecraft's transition into retirement, which began on October 30 with NASA's announcement that Kepler had run out of fuel and could no longer conduct science. 46)

- Coincidentally, Kepler's "goodnight" falls on the same date as the 388-year anniversary of the death of its namesake, German astronomer Johannes Kepler, who discovered the laws of planetary motion and passed away on Nov. 15, 1630.

- The final commands were sent over NASA's Deep Space Network from Kepler's operations center LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado in Boulder. LASP runs the spacecraft's operations on behalf of NASA and Ball Aerospace & Technologies Corporation in Boulder, Colorado.

- Kepler's team disabled the safety modes that could inadvertently turn systems back on, and severed communications by shutting down the transmitters. Because the spacecraft is slowly spinning, the Kepler team had to carefully time the commands so that instructions would reach the spacecraft during periods of viable communication. The team will monitor the spacecraft to ensure that the commands were successful. The spacecraft is now drifting in a safe orbit around the Sun, 94 million miles away from Earth.

- The data Kepler collected over the course of more than nine years in operation will be mined for exciting discoveries for many years to come.

- NASA's Ames Research Center in California's Silicon Valley manages the Kepler and K2 missions for NASA's Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from LASP.

October 30, 2018: After nine years in deep space collecting data that indicate our sky to be filled with billions of hidden planets – more planets even than stars – NASA’s Kepler space telescope has run out of fuel needed for further science operations. NASA has decided to retire the spacecraft within its current, safe orbit, away from Earth. Kepler leaves a legacy of more than 2,600 planet discoveries from outside our solar system, many of which could be promising places for life. 47)


Figure 29: This illustration depicts NASA's exoplanet hunter, the Kepler space telescope. The agency announced on 30 October 2018, that Kepler has run out of fuel and is being retired within its current and safe orbit, away from Earth. Kepler leaves a legacy of more than 2,600 exoplanet discoveries (image credit: NASA/Wendy Stenzel/Daniel Rutter)

- "As NASA's first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate in Washington. "Not only did it show us how many planets could be out there, it sparked an entirely new and robust field of research that has taken the science community by storm. Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars.”

- Kepler has opened our eyes to the diversity of planets that exist in our galaxy. The most recent analysis of Kepler’s discoveries concludes that 20 to 50 percent of the stars visible in the night sky are likely to have small, possibly rocky, planets similar in size to Earth, and located within the habitable zone of their parent stars. That means they’re located at distances from their parent stars where liquid water – a vital ingredient to life as we know it – might pool on the planet surface.

- The most common size of planet Kepler found doesn’t exist in our solar system – a world between the size of Earth and Neptune – and we have much to learn about these planets. Kepler also found nature often produces jam-packed planetary systems, in some cases with so many planets orbiting close to their parent stars that our own inner solar system looks sparse by comparison.

- "When we started conceiving this mission 35 years ago we didn't know of a single planet outside our solar system," said the Kepler mission's founding principal investigator, William Borucki, now retired from NASA’s Ames Research Center in California’s Silicon Valley. "Now that we know planets are everywhere, Kepler has set us on a new course that's full of promise for future generations to explore our galaxy."

- Launched on March 6, 2009, the Kepler space telescope combined cutting-edge techniques in measuring stellar brightness with the largest digital camera outfitted for outer space observations at that time. Originally positioned to stare continuously at 150,000 stars in one star-studded patch of the sky in the constellation Cygnus, Kepler took the first survey of planets in our galaxy and became the agency's first mission to detect Earth-size planets in the habitable zones of their stars.

- "The Kepler mission was based on a very innovative design. It was an extremely clever approach to doing this kind of science," said Leslie Livesay, director for astronomy and physics at NASA’s Jet Propulsion Laboratory, who served as Kepler project manager during mission development. "There were definitely challenges, but Kepler had an extremely talented team of scientists and engineers who overcame them.”

- Four years into the mission, after the primary mission objectives had been met, mechanical failures temporarily halted observations. The mission team was able to devise a fix, switching the spacecraft’s field of view roughly every three months. This enabled an extended mission for the spacecraft, dubbed K2, which lasted as long as the first mission and bumped Kepler's count of surveyed stars up to more than 500,000.

- The observation of so many stars has allowed scientists to better understand stellar behaviors and properties, which is critical information in studying the planets that orbit them. New research into stars with Kepler data also is furthering other areas of astronomy, such as the history of our Milky Way galaxy and the beginning stages of exploding stars called supernovae that are used to study how fast the universe is expanding. The data from the extended mission were also made available to the public and science community immediately, allowing discoveries to be made at an incredible pace and setting a high bar for other missions. Scientists are expected to spend a decade or more in search of new discoveries in the treasure trove of data Kepler provided.

- "We know the spacecraft's retirement isn't the end of Kepler's discoveries," said Jessie Dotson, Kepler's project scientist at NASA’s Ames Research Center in California’s Silicon Valley. "I'm excited about the diverse discoveries that are yet to come from our data and how future missions will build upon Kepler's results."

- Before retiring the spacecraft, scientists pushed Kepler to its full potential, successfully completing multiple observation campaigns and downloading valuable science data even after initial warnings of low fuel. The latest data, from Campaign 19, will complement the data from NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite, launched in April. TESS builds on Kepler's foundation with fresh batches of data in its search of planets orbiting some 200,000 of the brightest and nearest stars to the Earth, worlds that can later be explored for signs of life by missions, such as NASA’s James Webb Space Telescope.

- NASA's Ames Research Center in California's Silicon Valley manages the Kepler and K2 missions for NASA's Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation in Boulder, Colorado, operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

• September 28, 2018: NASA’s Kepler team has received data showing that the spacecraft’s ability to point precisely has degraded. In order to preserve high-value science data collected during its latest observation campaign, the Kepler team has placed the spacecraft in a stable, no-fuel-use sleep mode. 48)

- During Kepler’s allotted Deep Space Network time, scheduled to begin October 10, the Kepler team will “wake up” the spacecraft and direct it to point its large antenna back to Earth and transmit the science data home. Due to uncertainties about the remaining available fuel, there is no guarantee that NASA will be able to download the science data. If successful the Kepler team will attempt to start the next observing campaign with the remaining fuel.

- NASA anticipates that the spacecraft will soon run out of fuel, but it remains unclear how much remains. NASA’s goal is to collect and downlink as much science data as possible while the spacecraft remains viable.

- Kepler’s latest observing campaign, Campaign 19, started on August 29 after the spacecraft’s configuration had been modified in order to adapt to a change in thruster performance. Over the following 27 days, Kepler observed more than 30,000 stars and galaxies in the constellation of Aquarius. The stars included dozens of known and suspected exoplanet systems — including the well-known TRAPPIST-1 system with its seven Earth-sized planets.

- As engineers work to preserve the data stored onboard the spacecraft, scientists are continuing to mine existing data already on the ground. A recent notable find is Wolf 503b, a nearby super-Earth-size planet orbiting a bright star. At approximately twice the size of Earth, Wolf 503b is representative of the most common size of planet Kepler found in the galaxy. However, since there are no planets this size in our own solar system, we have a lot left to learn about planets this size. Since Wolf 503b is nearby and orbits a bright star, it is particularly well suited for subsequent observations with other telescopes that promise to help unravel the mysteries of what planets this size are like.

- Launched in March 2009, NASA’s first planet-hunter has confirmed more than 2,600 planets beyond the solar system.

• 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. 49) 50)

- 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 30: 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.