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TESS (Transiting Exoplanet Survey Satellite)

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TESS is a space telescope in NASA's Explorer program, designed to search for extrasolar planets using the transit method. The primary mission objective for TESS is to survey the brightest stars near the Earth for transiting exoplanets over a two-year period. The TESS project will use an array of wide-field cameras to perform an all-sky survey. It will scan nearby stars for exoplanets. 1) 2) 3)

In the first-ever spaceborne all-sky transit survey, TESS will identify planets ranging from Earth-sized to gas giants, orbiting a wide range of stellar types and orbital distances. The principal goal of the TESS mission is to detect small planets with bright host stars in the solar neighborhood, so that detailed characterizations of the planets and their atmospheres can be performed.

TESS will monitor the brightnesses of more than 200,000 stars during a two year mission, searching for temporary drops in brightness caused by planetary transits. Transits occur when a planet's orbit carries it directly in front of its parent star as viewed from Earth. TESS is expected to catalog more than 1,500 transiting exoplanet candidates, including a sample of ~500 Earth-sized and ‘Super Earth’ planets, with radii less than twice that of the Earth. TESS will detect small rock-and-ice planets orbiting a diverse range of stellar types and covering a wide span of orbital periods, including rocky worlds in the habitable zones of their host stars.

The lead institution for TESS is MIT (Massachusetts Institute of Technology), with George Ricker as PI (Principal Investigator). The MIT/LL (Lincoln Laboratory) is responsible for the cameras, including the lens assemblies, detector assemblies, lens hoods, and camera mount. NASA/GSFC (Goddard Space Flight Center) provides project management, systems engineering, and safety and mission assurance. Orbital ATK (OA) builds and operates the spacecraft. The mission is operated from the OA Mission Operations Center.

The TESS Science Center, which analyzes the science data and organizes the co-investigators, collaborators, and working groups (with members from many institutions) is a partnership among MIT's Physics Department and Kavli Institute for Astrophysics and Space Research, the SAO (Smithsonian Astrophysical Observatory), and the NASA Ames Research Center. The raw and processed data are archived at the Mikulski Archive for Space Telescopes, at the Space Telescope Science Institute.

Figure 1: This animation shows how a dip in the observed brightness of a star may indicate the presence of a planet passing in front of it, an occurrence known as a transit (image credit: NASA/GSFC)

Some background: TESS is a NASA-based mission, selected in 2013 as an astrophysics mission in the Explorers Program. TESS has a long history, beginning as a small, privately funded mission in 2006. It started with financial backing from private companies, including Google, the Kavli Foundation, and donors at MIT. This all changed in 2008, when MIT proposed TESS as an official NASA astrophysics mission, re-structuring it as a SMEX (Small Explorer) Class Mission. After not being selected in this competitive process for NASA resources, TESS proposed again in 2010 as a NASA Explorer (EX) Class Mission. TESS is the first of this new classification of Explorer missions. In 2013, TESS was successful in the proposal process and NASA began the development of the project. MIT's Kavli Institute of Technology for Astrophysics (MKI) has remained as an original partner in the current TESS mission, joining NASA in the next search for new worlds. 4)

TESS will concentrate on stars less than 300 light-years away and 30-100 times brighter than those surveyed by the Kepler satellite; thus,TESS planets should be far easier to characterize with follow-up observations. The brightness of these target stars will allow researchers to use spectroscopy, the study of the absorption and emission of light, to determine a planet’s mass, density and atmospheric composition. Water, and other key molecules, in its atmosphere can give us hints about a planets’ capacity to harbor life. These follow-up observations will provide refined measurements of the planet masses, sizes, densities, and atmospheric properties. 5)

TESS will provide prime targets for further, more detailed characterization with the James Webb Space Telescope (JWST), as well as other large ground-based and space-based telescopes of the future. TESS's legacy will be a catalog of the nearest and brightest stars hosting transiting exoplanets, which will comprise the most favorable targets for detailed investigations in the coming decades.

The Kepler project has provided ground-breaking new insights into the population of exoplanets in our galaxies; among the discoveries made using data from Kepler is the fact that the most common members of the exoplanet family are Earths and Super-Earths. However, the majority of exoplanets found by Kepler orbit faraway, faint stars. This, combined with the relatively small size of Earths and Super-Earths, means that there is currently a dearth of such planets that can be characterized with follow-up observations.

“TESS is opening a door for a whole new kind of study,” said Stephen Rinehart, TESS project scientist at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, which manages the mission. “We’re going to be able study individual planets and start talking about the differences between planets. The targets TESS finds are going to be fantastic subjects for research for decades to come. It’s the beginning of a new era of exoplanet research.”

Through the TESS Guest Investigator Program, the worldwide scientific community will be able to participate in investigations outside of TESS’s core mission, enhancing and maximizing the science return from the mission in areas ranging from exoplanet characterization to stellar astrophysics and solar system science (Ref. 6).

“I don’t think we know everything TESS is going to accomplish,” Rinehart said. “To me, the most exciting part of any mission is the unexpected result, the one that nobody saw coming.”

TESS is designed to:

• Focus on Earth and Super-Earth size planets

• Cover 400 X larger sky area than Kepler

• Span stellar spectral types of F5 to M5

Transiting exoplanets allow the project to observe the following for those planets that transit nearby bright stars:

• Fundamental properties: mass, radius, orbit

• Dynamics: planet-planet interactions, mutual inclinations, moons, tides

• Atmospheric composition + structure: transmission spectrum, emission spectrum, albedo, phase function, clouds, winds.

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Figure 2: Left: Sizes and orbital periods of planets with host stars brighter than J = 10. Right: Currently known planets, including those from the Kepler and CoRoT missions as well as ground-based surveys. Figure on the right now including the simulated population of TESS exoplanet detections (image credit: NASA)

TESS will tile the sky with 26 observation sectors:

• At least 27 days staring at each 24° x 96° sector

• Brightest 100,000 stars at 1-minute cadence

• Full frame images with 30-minute cadence

• Map Northern hemisphere in first year

• Map Southern hemisphere in second year

• Sectors overlap at ecliptic poles for sensitivity to smaller and longer period planets in JWST CVZ (Continuous Viewing Zone).

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Figure 3: Illustration of the TESS (Transiting Exoplanet Survey Telescope) in front of a lava planet orbiting its host star. TESS will identify thousands of potential new planets for further study and observation (image credit: NASA/GSFC) 6)




Spacecraft:

The TESS mission is based on Orbital's LEOStar-2 platform, a flexible, high-performance spacecraft for space and Earth science, remote sensing and other applications. LEOStar-2 can accommodate various instrument interfaces, deliver up to 2 kW orbit average payload power, and support payloads up to 500 kg. Performance options include redundancy, propulsion capability, high data rate communications, and high-agility/high-accuracy pointing. TESS will be the eighth LEOStar-2 based spacecraft built for NASA. Previous missions include SORCE, GALEX, AIM, NuSTAR and the OCO-2 spacecraft.

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Figure 4: Illustration of Orbital ATK LEOStar-2 minisatellite (image credit: Orbital ATK)

The LEOStar-2 bus has a three-axis controlled, zero-momentum attitude control system, and two deployed solar array wings. The total observatory power draw is estimated to be 290 W, and the solar arrays are capable of producing 415 W. To achieve fine pointing, the spacecraft uses four reaction wheels and high-precision quaternions produced by the science cameras. The transmitter has a body-fixed high-gain antenna with a diameter of 0.7 m, a power of 2 W and a data rate of 100 Mbit/s. This is sufficient to downlink the science data during 4 hr intervals at each perigee.

Spacecraft bus

Heritage Orbital LEOStar-2 spacecraft bus

Launch mass

325 kg

Spacecraft size (deployed)

3.9 m x 1.2 m x 1.5 m

Redundancy

Selective

Solar arrays

400 W (EOL), Two wing solar array, fixed and articulating modes

Stabilization

3-axis zero momentum bias via 4 hydrazine thrusters, four wheel fine-pointing ACS (Attitude Control Subsystem)

Pointing accuracy

3.2 arcsec control, 2.7 arcsec knowledge

Propulsion subsystem

Mono-propellant propulsion subsystem

TCS (Thermal Control Subsystem)

Passive thermal control

Mission life

2 years

RF communications

Ka-band 100 Mbit/s science downlink

Table 1: Overview of spacecraft parameters 7)

DHU (Data Handling Unit): The DHU is a Space Micro Image Processing Computer (IPC-7000) which consists of six boards: an IPC (Image Processing Computer), which contains two Virtex-7 FPGAs (Field Programmable Gate Arrays) that serve as interfaces to the four cameras and perform high-speed data processing; a Proton 400 k single board computer, which is responsible for commanding, communicating with the spacecraft master avionics unit, and interfacing with the Ka-band transmitter; two 192 GB SSB (Solid-State Buffer) cards for mass data storage; an analog I/O power switch board to control instrument power; and a power supply board for the DHU.

The CCDs (Charge Coupled Devices) produce a continuous stream of images with an exposure time of 2 seconds. These are received by the FPGAs on the IPC, and summed into consecutive groups of 60, giving an effective exposure time of 2 minutes. During science operations, the DHU performs real-time processing of data from the four cameras, converting CCD images into the data products required for ground post-processing. A primary data product is a collection of subarrays (nominally 10 x 10 pixels) centered on preselected target stars. The Proton400 k extracts these subarrays from each 2 min summed image, compresses them and stores them in the SSB prior to encapsulation as CCSDS packets for the Ka-band transmitter. Full frame images are also stacked every 30 minutes and stored in the SSB. Data from the SSB are downlinked every 13.7 days at perigee.

At perigee, science operations are interrupted for no more than 16 hours to point TESS 's antenna toward Earth, downlink data, and resume observing. This includes a nominal 4 hr period for Ka-band science data downlink using NASA's DSN (Deep Space Network). In addition, momentum unloading is occasionally needed due to the ~1.5 N m of angular momentum build-up induced by solar radiation pressure. For this purpose TESS uses its hydrazine thrusters.

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Figure 5: Left: Diagram illustrating the orientations of the four TESS cameras, lens hoods, and mounting platform. Right: Artist's conception of the TESS spacecraft and payload (image credit: Orbital ATK, TESS Team)


Development status:

• February 15, 2018: NASA's TESS satellite has arrived in Florida to begin preparations for launch. TESS was delivered Feb. 12 aboard a truck from Orbital ATK in Dulles, Virginia, where it spent 2017 being assembled and tested. Over the next month, the spacecraft will be prepped for launch at Kennedy's Payload Hazardous Servicing Facility (PHSF). 8)

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Figure 6: TESS arrives at NASA’s Kennedy Space Center, where it will undergo final preparations for launch. Launch is scheduled for no earlier than April 16, pending range approval (image credit: NASA’s Kennedy Space Center)


Launch: The TESS spacecraft was launched on 18 April 2018 (22:51 UTC) from the Cape Canaveral Air Force Station in Florida, SLC-40 (Space Launch Complex-40). The launch provider was SpaceX using the Falcon-9 V1.1 launch vehicle. 9) 10) 11) 12) 13)

Following stage separation, SpaceX successfully landed Falcon 9’s first stage on the “Of Course I Still Love You” droneship in the Atlantic Ocean. — After TESS was released the satellite deployed its solar arrays, and it will take 60 days for the satellite to attain its proper orbit.

Orbit: HEO (Highly Elliptical Orbit) with a nominal perigee of 17 RE (Earth radii) equivalent to 108,000 km, and a nominal apogee of 59 RE or 373,000 km, inclination = 28.5º, period of 13.7 days in 2:1 resonance with the Moon's orbit.

The orbit remains above the Earth's radiation belts, leading to a relatively low-radiation environment with a mission total ionizing dose of <1 krad. The nearly constant thermal environment ensures that the CCDs will operate near -75ºC, with temperature variations <0.1ºC /hr for 90% of the orbit, and <2ºC/hr throughout the entire orbit (Ref. 3).

This orbit can be reached efficiently using a small supplemental propulsion system (ΔV ~3 km/s) augmented by a lunar gravity assist. The specific path to the orbit will depend on the launch date and launch vehicle. In a nominal scenario (illustrated in Figure 7), TESS is launched from Cape Canaveral into a parking orbit with an equatorial inclination of 28.5º. The apogee is raised to 400,000 km after two additional burns by the spacecraft hydrazine system, one at perigee of the first phasing orbit, and another at perigee of the second phasing orbit. An adjustment is made at third perigee, before a lunar flyby raises the ecliptic inclination to about 40º. A final period-adjust maneuver establishes the desired apogee and the 13.7 day period. The final orbit is achieved about 60 days after launch, and science operations begin soon afterward.

The orbital period and semimajor axis are relatively constant, with long-term exchanges of eccentricity and inclination over a period of order 8-12 years (driven by a Kozai-like mechanism) 14). There are also short-term oscillations with a period of six months caused by solar perturbations ( Figure 8). The orbit is stable on the time scale of decades, or more, and requires no propulsion for station-keeping. Table 2 lists a number of advantages of this type of orbit for TESS.

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Figure 7: Maneuvers and scenario for achieving the TESS mission orbit. PLEP (Post Lunar-Encounter Perigee) and PLEA (Post Lunar-Encounter Apogee), image credit: TESS Team

• Extended and unbroken observations: >300 hr orbit-1

• Thermal stability: <0.1ºC hr-1 (passive control)

• Earth/Moon stray light: ~106 times lower than in low-Earth orbit

• Low radiation levels: no South Atlantic anomaly or outer belt electrons

• Frequent launch windows: 20 days per lunation

• High data rates at perigee: ~100 Mbit s-1

• Excellent pointing stability: no drag or gravity gradient torques

• Simple operations: single 4 hr downlink & repoint every 2 weeks

• Long lifetime: several decades above GEO (>6.6 RE)

Table 2: Characteristics of the TESS spacecraft orbit and comparisons to a low-Earth orbit

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Figure 8: Calculated time variations in the elements of the nominal TESS mission orbit. The units of each curve are specified in the legend; AOP (Argument of Perigee), GEO (Geosynchronous Earth Orbit), image credit: TESS Team




Mission status:

• December 4, 2019: Using data from the TESS satellite, astronomers at the University of Maryland (UMD), in College Park, Maryland, have captured a clear start-to-finish image sequence of an explosive emission of dust, ice and gases during the close approach of comet 46P/Wirtanen in late 2018. This is the most complete and detailed observation to date of the formation and dissipation of a naturally-occurring comet outburst. The team members reported their results in the November 22 issue of The Astrophysical Journal Letters. 15) 16) 17)

- “TESS spends nearly a month at a time imaging one portion of the sky. With no day or night breaks and no atmospheric interference, we have a very uniform, long-duration set of observations,” said Tony Farnham, a research scientist in the UMD Department of Astronomy and the lead author of the research paper. “As comets orbit the Sun, they can pass through TESS’ field of view. Wirtanen was a high priority for us because of its close approach in late 2018, so we decided to use its appearance in the TESS images as a test case to see what we could get out of it. We did so and were very surprised!”

Figure 9: This animation shows an explosive outburst of dust, ice and gases from comet 46P/Wirtanen that occurred on September 26, 2018 and dissipated over the next 20 days. The images, from NASA’s TESS spacecraft, were taken every three hours during the first three days of the outburst (image credit: Farnham et al./NASA)

- “While TESS is a powerhouse for discovering planets orbiting nearby, bright stars, its observing strategy enables so much exciting additional science,” said TESS project scientist Padi Boyd of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Since the TESS data are rapidly made public through NASA’s Mikulski Archive for Space Telescopes (MAST), it’s exciting to see scientists identifying which data are of interest to them, and then doing all kinds of additional serendipitous science beyond exoplanets.”

- Normal comet activity is driven by sunlight vaporizing the ices near the surface of the nucleus, and the outflowing gases drag dust off the nucleus to form the coma. However, many comets are known to experience occasional spontaneous outbursts that can significantly, but temporarily increase the comet's activity. It is not currently known what causes outbursts, but they are related to the conditions on the comet's surface. A number of potential trigger mechanisms have been proposed, including a thermal event, in which a heat wave penetrates into a pocket of highly volatile ices, causing the ice to rapidly vaporize and produce an explosion of activity, and a mechanical event, where a cliff collapses, exposing fresh ice to direct sunlight. Thus, studies of the outburst behavior, especially in the early brightening stages that are difficult to capture, can help us understand the physical and thermal properties of the comet.

- Although Wirtanen came closest to Earth on December 16, 2018, the outburst occurred earlier in its approach, beginning on September 26, 2018. The initial brightening of the outburst occurred in two distinct phases, with an hour-long flash followed by a more gradual second stage that continued to grow brighter for another 8 hours. This second stage was likely caused by the gradual spreading of comet dust from the outburst, which causes the dust cloud to reflect more sunlight overall. After reaching peak brightness, the comet faded gradually over a period of more than two weeks. Because TESS takes detailed, composite images every 30 minutes, the team was able to view each phase in exquisite detail.

- “With 20 days’ worth of very frequent images, we were able to assess changes in brightness very easily. That’s what TESS was designed for, to perform its primary job as an exoplanet surveyor,” Farnham said. “We can’t predict when comet outbursts will happen. But even if we somehow had the opportunity to schedule these observations, we couldn’t have done any better in terms of timing. The outburst happened mere days after the observations started.”

- The team has generated a rough estimate of how much material may have been ejected in the outburst, about one million kilograms, which could have left a crater on the comet of around 20 meters (about 65 feet) across. Further analysis of the estimated particle sizes in the dust tail may help improve this estimate. Observing more comets will also help to determine whether multi-stage brightening is rare or commonplace in comet outbursts.

- TESS has also detected for the first time Wirtanen’s dust trail. Unlike a comet’s tail—the spray of gas and fine dust that follows behind a comet, growing as it approaches the sun—a comet’s trail is a field of larger debris that traces the comet’s orbital path as it travels around the sun. Unlike a tail, which changes direction as it is blown by the solar wind, the orientation of the trail stays more or less constant over time.

- “The trail more closely follows the orbit of the comet, while the tail is offset from it, as it gets pushed around by the sun’s radiation pressure. What’s significant about the trail is that it contains the largest material,” said Michael Kelley, an associate research scientist in the UMD Department of Astronomy and a co-author of the research paper. “Tail dust is very fine, a lot like smoke. But trail dust is much larger—more like sand and pebbles. We think comets lose most of their mass through their dust trails. When the Earth runs into a comet’s dust trail, we get meteor showers.”

- While the current study describes initial results, Farnham, Kelley and their colleagues look forward to further analyses of Wirtanen, as well as other comets in TESS’ field of view. “We also don’t know what causes natural outbursts and that’s ultimately what we want to find,” Farnham said. “There are at least four other comets in the same area of the sky where TESS made these observations, with a total of about 50 comets expected in the first two years’ worth of TESS data. There’s a lot that can come of these data.”

- TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.

• November 5, 2019: The glow of the Milky Way — our galaxy seen edgewise — arcs across a sea of stars in a new mosaic of the southern sky produced from a year of observations by NASA’s Transiting Exoplanet Survey Satellite (TESS). Constructed from 208 TESS images taken during the mission’s first year of science operations, completed on July 18, the southern panorama reveals both the beauty of the cosmic landscape and the reach of TESS’s cameras. 18)

- “Analysis of TESS data focuses on individual stars and planets one at a time, but I wanted to step back and highlight everything at once, really emphasizing the spectacular view TESS gives us of the entire sky,” said Ethan Kruse, a NASA Postdoctoral Program Fellow who assembled the mosaic at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Figure 10: NASA’s Transiting Exoplanet Survey Satellite (TESS) spent a year imaging the southern sky in its search for worlds beyond our solar system. Dive into a mosaic of these images to see what TESS has found so far (video credit: NASA’s Godard Space Flight Center)

- Within this scene, TESS has discovered 29 exoplanets, or worlds beyond our solar system, and more than 1,000 candidate planets astronomers are now investigating.

- TESS divided the southern sky into 13 sectors and imaged each one of them for nearly a month using four cameras, which carry a total of 16 charge-coupled devices (CCDs). Remarkably, the TESS cameras capture a full sector of the sky every 30 minutes as part of its search for exoplanet transits. Transits occur when a planet passes in front of its host star from our perspective, briefly and regularly dimming its light. During the satellite’s first year of operations, each of its CCDs captured 15,347 30-minute science images. These images are just a part of more than 20 terabytes of southern sky data TESS has returned, comparable to streaming nearly 6,000 high-definition movies.

- In addition to its planet discoveries, TESS has imaged a comet in our solar system, followed the progress of numerous stellar explosions called supernovae, and even caught the flare from a star ripped apart by a supermassive black hole. After completing its southern survey, TESS turned north to begin a year-long study of the northern sky.

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Figure 11: The plane of our Milky Way galaxy arcs across a starry landscape in this detail of the TESS southern sky mosaic [image credit: NASA/MIT/TESS and Ethan Kruse (USRA)]

• October 30, 2019: Using asteroseismic data from NASA's TESS (Transiting Exoplanet Survey Satellite) mission, an international team, led by Instituto de Astrofisica e Ciencias do Espaco (IA) researcher Tiago Campante (Porto, Portugal), studied the red-giant stars HD 212771 and HD 203949. These are the first detections of oscillations in previously known exoplanet-host stars by TESS. The result was published today in an article in The Astrophysical Journal. 19) 20)

- Tiago Campante (IA and Faculdade de Ciencias da Universidade do Porto - FCUP) explains that detecting these oscillations was only possible because: "TESS observations are precise enough to allow measuring the gentle pulsations at the surfaces of stars. These two fairly evolved stars also host planets, providing the ideal testbed for studies of the evolution of planetary systems."

- Having determined the physical properties of both stars, such as their mass, size and age, through asteroseismology, the authors then focused their attention on the evolutionary state of HD 203949. Their aim was to understand how its planet could have avoided engulfment, since the envelope of the star would have expanded well beyond the current planetary orbit during the red-giant phase of evolution.

- Co-author Vardan Adibekyan (IA and Universidade do Porto) comments: "This study is a perfect demonstration of how stellar and exoplanetary astrophysics are linked together. Stellar analysis seems to suggest that the star is too evolved to still host a planet at such a 'short' orbital distance, while from the exoplanet analysis we know that the planet is there!"

- By performing extensive numerical simulations, the team thinks that star-planet tides might have brought the planet inward from its original, wider orbit, placing it where we see it today.

- Adibekyan adds: "The solution to this scientific dilemma is hidden in the 'simple fact' that stars and their planets not only form but also evolve together. In this particular case, the planet managed to avoid engulfment."

- In the past decade, asteroseismology has had a significant impact on the study of solar-type and red-giant stars, which exhibit convection-driven, solar-like oscillations. These studies have advanced considerably with space-based observatories like CoRoT (CNES/ESA) and Kepler (NASA), and are set to continue in the next decade with TESS and PLATO (ESA).

- Tiago Campante explains that: "IA's involvement in TESS is at the level of the scientific coordination within the TESS Asteroseismic Science Consortium (TASC). TASC is a large and unique scientific collaboration, bringing together all relevant research groups and individuals from around the world who are actively engaged in research in the field of asteroseismology. Following in the footsteps of its successful predecessor, the Kepler Asteroseismic Science Consortium (KASC), TASC is based on a collaborative and transparent working-group structure, aimed at facilitating open collaboration between scientists."

• September 26, 2019: For the first time, NASA’s planet-hunting TESS watched a black hole tear apart a star in a cataclysmic phenomenon called a tidal disruption event. Follow-up observations by NASA’s Neil Gehrels Swift Observatory and other facilities have produced the most detailed look yet at the early moments of one of these star-destroying occurrences. 21)

- “TESS data let us see exactly when this destructive event, named ASASSN-19bt, started to get brighter, which we’ve never been able to do before,” said Thomas Holoien, a Carnegie Fellow at the Carnegie Observatories in Pasadena, California. “Because we identified the tidal disruption quickly with the ground-based All-Sky Automated Survey for Supernovae (ASAS-SN), we were able to trigger multiwavelength follow-up observations in the first few days. The early data will be incredibly helpful for modeling the physics of these outbursts.”

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Figure 12: This illustration shows a tidal disruption, which occurs when a passing star gets too close to a black hole and is torn apart into a stream of gas. Some of the gas eventually settles into a structure around the black hole called an accretion disk (image credit: NASA's Goddard Space Flight Center)

Figure 13: When a star strays too close to a black hole, intense tides break it apart into a stream of gas. The tail of the stream escapes the system, while the rest of it swings back around, surrounding the black hole with a disk of debris. This video includes images of a tidal disruption event called ASASSN-19bt taken by NASA’s TESS and Swift missions, as well as an animation showing how the event unfolded (video credit: NASA's Goddard Space Flight Center)

- A paper describing the findings, led by Holoien, was published in the Sept. 27, 2019, issue of The Astrophysical Journal and is now available online. 22)

- ASAS-SN, a worldwide network of 20 robotic telescopes headquartered at Ohio State University (OSU) in Columbus, discovered the event on Jan. 29. Holoien was working at the Las Campanas Observatory in Chile when he received the alert from the project’s South Africa instrument. Holoien quickly trained two Las Campanas telescopes on ASASSN-19bt and then requested follow-up observations by Swift, ESA’s (European Space Agency’s) XMM-Newton and ground-based 1-meter telescopes in the global Las Cumbres Observatory network.

- TESS, however, didn’t need a call to action because it was already looking at the same area. The planet hunter monitors large swaths of the sky, called sectors, for 27 days at a time. This lengthy view allows TESS to observe transits, periodic dips in a star’s brightness that may indicate orbiting planets.

- ASAS-SN began spending more time looking at TESS sectors when the satellite started science operations in July 2018. Astronomers anticipated TESS could catch the earliest light from short-lived stellar outbursts, including supernovae and tidal disruptions. TESS first saw ASASSN-19bt on Jan. 21, over a week before the event was bright enough for ASAS-SN to detect it. However, the satellite only transmits data to Earth every two weeks, and once received they must be processed at NASA’s Ames Research Center in Silicon Valley, California. So the first TESS data on the tidal disruption were not available until March 13. This is why obtaining early follow-up observations of these events depends on coordination by ground-based surveys like ASAS-SN.

- Fortunately, the disruption also occurred in TESS’s southern continuous viewing zone, which was always in sight of one of the satellite’s four cameras. (TESS shifted to monitoring the northern sky at the end of July.) ASASSN-19bt’s location allowed Holoien and his colleagues to follow the event across several sectors. If it had occurred outside this zone, TESS might have missed the beginning of the outburst.

- “The early TESS data allow us to see light very close to the black hole, much closer than we’ve been able to see before,” said Patrick Vallely, a co-author and National Science Foundation Graduate Research Fellow at OSU. “They also show us that ASASSN-19bt’s rise in brightness was very smooth, which helps us tell that the event was a tidal disruption and not another type of outburst, like from the center of a galaxy or a supernova.”

- Holoien’s team used UV data from Swift — the earliest yet seen from a tidal disruption — to determine that the temperature dropped by about 50%, from around 71,500 to 35,500 degrees Fahrenheit (40,000 to 20,000 º Celsius), over a few days. It’s the first time such an early temperature decrease has been seen in a tidal disruption before, although a few theories have predicted it, Holoien said.

- More typical for these kinds of events was the low level of X-ray emission seen by both Swift and XMM-Newton. Scientists don’t fully understand why tidal disruptions produce so much UV emission and so few X-rays.

- “People have suggested multiple theories — perhaps the light bounces through the newly created debris and loses energy, or maybe the disk forms further from the black hole than we originally thought and the light isn’t so affected by the object’s extreme gravity,” said S. Bradley Cenko, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “More early-time observations of these events may help us answer some of these lingering questions.”

- Astronomers think the supermassive black hole that generated ASASSN-19bt weighs around 6 million times the Sun’s mass. It sits at the center of a galaxy called 2MASX J07001137-6602251 located around 375 million light-years away in the constellation Volans. The destroyed star may have been similar in size to our Sun.

- Tidal disruptions are incredibly rare, occurring once every 10,000 to 100,000 years in a galaxy the size of our own Milky Way. Supernovae, by comparison, happen every 100 years or so. In total, astronomers have observed only about 40 tidal disruptions so far, and scientists predicted TESS would see only one or two in its initial two-year mission.

- “For TESS to observe ASASSN-19bt so early in its tenure, and in the continuous viewing zone where we could watch it for so long, is really quite extraordinary,” said Padi Boyd, the TESS project scientist at Goddard. “Future collaborations with observatories around the world and in orbit will help us learn even more about the different outbursts that light up the cosmos.”

- TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.

- NASA's Goddard Space Flight Center manages the Swift mission in collaboration with Penn State in University Park, the Los Alamos National Laboratory in New Mexico and Northrop Grumman Innovation Systems in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory of the University College London in the United Kingdom, Brera Observatory and ASI.

• July 29, 2019: NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS), has discovered three new worlds — one slightly larger than Earth and two of a type not found in our solar system — orbiting a nearby star. The planets straddle an observed gap in the sizes of known planets and promise to be among the most curious targets for future studies. 23)

- TESS Object of Interest (TOI) 270 is a faint, cool star more commonly identified by its catalog name: UCAC4 191-004642. The M-type dwarf star is about 40% smaller than the Sun in both size and mass, and it has a surface temperature about one-third cooler than the Sun’s. The planetary system lies about 73 light-years away in the southern constellation of Pictor.

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Figure 14: This infographic illustrates key features of the TOI 270 system, located about 73 light-years away in the southern constellation Pictor. The three known planets were discovered by NASA’s Transiting Exoplanet Survey Satellite through periodic dips in starlight caused by each orbiting world. Insets show information about the planets, including their relative sizes, and how they compare to Earth. Temperatures given for TOI 270’s planets are equilibrium temperatures, calculated without the warming effects of any possible atmospheres (image credit: NASA’s Goddard Space Flight Center/Scott Wiessinger)

- “This system is exactly what TESS was designed to find — small, temperate planets that pass, or transit, in front of an inactive host star, one lacking excessive stellar activity, such as flares,” said lead researcher Maximilian Günther, a Torres Postdoctoral Fellow at the Massachusetts Institute of Technology’s (MIT) Kavli Institute for Astrophysics and Space Research in Cambridge. “This star is quiet and very close to us, and therefore much brighter than the host stars of comparable systems. With extended follow-up observations, we’ll soon be able to determine the make-up of these worlds, establish if atmospheres are present and what gases they contain, and more.”

- A paper describing the system was published in the journal Nature Astronomy and is now available online. 24)

- The innermost planet, TOI 270 b, is likely a rocky world about 25% larger than Earth. It orbits the star every 3.4 days at a distance about 13 times closer than Mercury orbits the Sun. Based on statistical studies of known exoplanets of similar size, the science team estimates TOI 270 b has a mass around 1.9 times greater than Earth’s.

- Due to its proximity to the star, planet b is an oven-hot world. Its equilibrium temperature — that is, the temperature based only on energy it receives from the star, which ignores additional warming effects from a possible atmosphere — is around 490 º Fahrenheit (254ºC).

- The other two planets, TOI 270 c and d, are, respectively, 2.4 and 2.1 times larger than Earth and orbit the star every 5.7 and 11.4 days. Although only about half its size, both may be similar to Neptune in our solar system, with compositions dominated by gases rather than rock, and they likely weigh around 7 and 5 times Earth’s mass, respectively.

Figure 15: Compare and contrast worlds in the TOI 270 system with these illustrations of each planet. Temperatures given for TOI 270 planets are equilibrium temperatures, calculated without taking into account the warming effects of any possible atmospheres (image credit: NASA’s Goddard Space Flight Center)

- All of the planets are expected to be tidally locked to the star, which means they only rotate once every orbit and keep the same side facing the star at all times, just as the Moon does in its orbit around Earth.

- Planet c and d might best be described as mini-Neptunes, a type of planet not seen in our own solar system. The researchers hope further exploration of TOI 270 may help explain how two of these mini-Neptunes formed alongside a nearly Earth-size world.

- “An interesting aspect of this system is that its planets straddle a well-established gap in known planetary sizes,” said co-author Fran Pozuelos, a postdoctoral researcher at the University of Liège in Belgium. “It is uncommon for planets to have sizes between 1.5 and two times that of Earth for reasons likely related to the way planets form, but this is still a highly controversial topic. TOI 270 is an excellent laboratory for studying the margins of this gap and will help us better understand how planetary systems form and evolve.”

- Günther’s team is particularly interested in the outermost planet, TOI 270 d. The team estimates the planet’s equilibrium temperature to be about 150º Fahrenheit (66º C). This makes it the most temperate world in the system — and as such, a rarity among known transiting planets.

- "TOI 270 is perfectly situated in the sky for studying the atmospheres of its outer planets with NASA's future James Webb Space Telescope," said co-author Adina Feinstein, a doctoral student at the University of Chicago. "It will be observable by Webb for over half a year, which could allow for really interesting comparison studies between the atmospheres of TOI 270 c and d."

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Figure 16: The TOI 270 system is so compact that the orbits of Jupiter and its moons in our own solar system offer the closest reasonable comparison, as illustrated here (image credit: NASA/GSFC (Goddard Space Flight Center))

- The team hopes further research may reveal additional planets beyond the three now known. If planet d has a rocky core covered by a thick atmosphere, its surface would be too warm for the presence of liquid water, considered a key requirement for a potentially habitable world. But follow-up studies may discover additional rocky planets at slightly greater distances from the star, where cooler temperatures could allow liquid water to pool on their surfaces.

- TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.

• July 25, 2019: NASA’s TESS (Transiting Exoplanet Survey Satellite) has discovered 21 planets outside our solar system and captured data on other interesting events occurring in the southern sky during its first year of science. TESS has now turned its attention to the Northern Hemisphere to complete the most comprehensive planet-hunting expedition ever undertaken. 25)

- TESS began hunting for exoplanets (or worlds orbiting distant stars) in the southern sky in July of 2018, while also collecting data on supernovae, black holes and other phenomena in its line of sight. Along with the planets TESS has discovered, the mission has identified over 850 candidate exoplanets that are waiting for confirmation by ground-based telescopes.

- “The pace and productivity of TESS in its first year of operations has far exceeded our most optimistic hopes for the mission,” said George Ricker, TESS’s principal investigator at the Massachusetts Institute of Technology in Cambridge. “In addition to finding a diverse set of exoplanets, TESS has discovered a treasure trove of astrophysical phenomena, including thousands of violently variable stellar objects.”

- To search for exoplanets, TESS uses four large cameras to watch a 24-by-96-degree section of the sky for 27 days at a time. Some of these sections overlap, so some parts of the sky are observed for almost a year. TESS is concentrating on stars closer than 300 light-years from our solar system, watching for transits, which are periodic dips in brightness caused by an object, like a planet, passing in front of the star.

Figure 17: Here are highlights from TESS's first year of science operations. All exoplanet animations are illustrations. To search for exoplanets, TESS uses four large cameras to watch a 24 x 96 degree section of the sky for 27 days at a time. Some of these sections overlap, so some parts of the sky are observed for almost a year. TESS is concentrating on stars closer than 300 light-years from our solar system, watching for transits, which are periodic dips in brightness caused by an object, like a planet, passing in front of the star (video credit: NASA/GSFC, Published on 25 July 2019)

- On July 18, the southern portion of the survey was completed and the spacecraft turned its cameras to the north. When it completes the northern section in 2020, TESS will have mapped over three quarters of the sky.

- “Kepler discovered the amazing result that, on average, every star system has a planet or planets around it,” said Padi Boyd, TESS project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “TESS takes the next step. If planets are everywhere, let’s find those orbiting bright, nearby stars because they’ll be the ones we can now follow up with existing ground and space-based telescopes, and the next generation of instruments for decades to come.”

- Here are a few of the interesting objects and events TESS saw during its first year.

Exoplanets

- To qualify as an exoplanet candidate, an object must make at least three transits in the TESS data, and then pass through several additional checks to make sure the transits were not a false positive caused by an eclipse or companion star, but may in fact be an exoplanet. Once a candidate is identified, astronomers deploy a large network of ground-based telescopes to confirm it.

- “The team is currently focused on finding the best candidates to confirm by ground-based follow-up,” said Natalia Guerrero, who manages the team in charge of identifying exoplanet candidates at MIT. “But there are many more potential exoplanet candidates in the data yet to be analyzed, so we’re really just seeing the tip of the iceberg here. TESS has only scratched the surface.”

- The planets TESS has discovered so far range from a world 80% the size of Earth to ones comparable to or exceeding the sizes of Jupiter and Saturn. Like Kepler, TESS is finding many planets smaller in size than Neptune, but larger than Earth.

- While NASA is striving to put astronauts on some of our nearest neighbors — the Moon and Mars — in order to understand more about the planets in our own solar system, follow-up observations with powerful telescopes of the planets TESS discovers will enable us to better understand how Earth and the solar system formed.

- With TESS’s data, scientists using current and future observatories, like the James Webb Space Telescope, will be able to study other aspects of exoplanets, like the presence and composition of any atmosphere, which would impact the possibility of developing life.

Comets

- Before science operations started, TESS snapped clear images of a newly discovered comet in our solar system. During on-orbit instrument testing, the satellite’s cameras took a series of images that captured the motion of C/2018 N1, a comet found on June 29 by NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).

- TESS captured data on similar objects outside the solar system as well.

Exocomets

- Data from the mission were also used to identify transits by comets orbiting another star: Beta Pictoris, located 63 light-years away. Astronomers were able to find three comets that were too small to be planets and had detectable tails, the first identification of its type in visible light.

Supernovae

- Because TESS spends nearly a month looking in the same location, it can capture data on stellar events, like supernovae, as they begin. During its first months of science operations, TESS spotted six supernovae occurring in distant galaxies that were later discovered by ground-based telescopes.

- Scientists hope to use these types of observations to better understand the origins of a specific kind of explosion known as a Type Ia supernova.

- Type Ia supernovae occur either in star systems where one white dwarf draws gas from another star or when two white dwarfs merge. Astronomers don’t know which case is more common, but with data from TESS, they’ll have a clearer understanding of the origins of these cosmic blasts.

- Type Ia supernovae are a class of objects called a “standard candle,” meaning astronomers know how luminous they are and can use them to calculate quantities like how quickly the universe is expanding. TESS data will help them understand differences between Type Ia supernovae created in both circumstances, which could have a large impact on how we understand events happening billions of light-years away and, ultimately, the fate of the universe.

• June 27, 2019: NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered a world between the sizes of Mars and Earth orbiting a bright, cool, nearby star. The planet, called L 98-59b, marks the tiniest planet discovered by TESS to date. 26)

- Two other worlds orbit the same star. While all three planets’ sizes are known, further study with other telescopes will be needed to determine if they have atmospheres and, if so, which gases are present. The L 98-59 worlds nearly double the number of small exoplanets — that is, planets beyond our solar system — that have the best potential for this kind of follow-up.

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Figure 18: The three planets discovered in the L98-59 system by NASA’s TESS mission are compared to Mars and Earth in order of increasing size in this illustration (image credit: NASA/GSFC)

- “The discovery is a great engineering and scientific accomplishment for TESS,” said Veselin Kostov, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the SETI Institute in Mountain View, California. “For atmospheric studies of small planets, you need short orbits around bright stars, but such planets are difficult to detect. This system has the potential for fascinating future studies.” 27)

- L 98-59b is around 80% Earth’s size and about 10% smaller than the previous record holder discovered by TESS. Its host star, L 98-59, is an M dwarf about one-third the mass of the Sun and lies about 35 light-years away in the southern constellation Volans. While L 98-59b is a record for TESS, even smaller planets have been discovered in data collected by NASA’s Kepler satellite, including Kepler-37b, which is only 20% larger than the Moon.

- The two other worlds in the system, L 98-59c and L 98-59d, are respectively around 1.4 and 1.6 times Earth’s size. All three were discovered by TESS using transits, periodic dips in the star’s brightness caused when each planet passes in front of it.

- TESS monitors one 24-by-96-degree region of the sky, called a sector, for 27 days at a time. When the satellite finishes its first year of observations in July, the L 98-59 system will have appeared in seven of the 13 sectors that make up the southern sky. Kostov’s team hopes this will allow scientists to refine what’s known about the three confirmed planets and search for additional worlds.

- “If you have more than one planet orbiting in a system, they can gravitationally interact with each other,” said Jonathan Brande, a co-author and astrophysicist at Goddard and the University of Maryland, College Park. “TESS will observe L 98-59 in enough sectors that it may be able to detect planets with orbits around 100 days. But if we get really lucky, we might see the gravitational effects of undiscovered planets on the ones we currently know.”

- M dwarfs like L 98-59 account for three-quarters of our Milky Way galaxy’s stellar population. But they are no larger than about half the Sun’s mass and are much cooler, with surface temperatures less than 70% of the Sun’s. Other examples include TRAPPIST-1, which hosts a system of seven Earth-size planets, and Proxima Centauri, our nearest stellar neighbor, which has one confirmed planet. Because these small, cool stars are so common, scientists want to learn more about the planetary systems that form around them.

- L 98-59b, the innermost world, orbits every 2.25 days, staying so close to the star it receives as much as 22 times the amount of energy Earth receives from the Sun. The middle planet, L 98-59c, orbits every 3.7 days and experiences about 11 times as much radiation as Earth. L 98-59d, the farthest planet identified in the system so far, orbits every 7.5 days and is blasted with around four times the radiant energy as Earth.

- None of the planets lie within the star’s “habitable zone,” the range of distances from the star where liquid water could exist on their surfaces. However, all of them occupy what scientists call the Venus zone, a range of stellar distances where a planet with an initial Earth-like atmosphere could experience a runaway greenhouse effect that transforms it into a Venus-like atmosphere. Based on its size, the third planet could be either a Venus-like rocky world or one more like Neptune, with a small, rocky core cocooned beneath a deep atmosphere.

- One of TESS’s goals is to build a catalog of small, rocky planets on short orbits around very bright, nearby stars for atmospheric study by NASA's upcoming James Webb Space Telescope. Four of the TRAPPIST-1 worlds are prime candidates, and Kostov’s team suggests the L 98-59 planets are as well.

- The TESS mission feeds our desire to understand where we came from and whether we’re alone in the universe.

- "If we viewed the Sun from L 98-59, transits by Earth and Venus would lead us to think the planets are almost identical, but we know they’re not,” said Joshua Schlieder, a co-author and an astrophysicist at Goddard. “We still have many questions about why Earth became habitable and Venus did not. If we can find and study similar examples around other stars, like L 98-59, we can potentially unlock some of those secrets.”

• April 15, 2019: NASA’s TESS (Transiting Exoplanet Survey Satellite) has discovered its first Earth-size world. The planet, HD 21749c, is about 89% Earth’s diameter. It orbits HD 21749, a K-type star with about 70% of the Sun’s mass located 53 light-years away in the southern constellation Reticulum, and is the second planet TESS has identified in the system. The new world is likely rocky and circles very close to its star, completing one orbit in just under eight days. The planet is likely very hot, with surface temperatures perhaps as high as 800º F (427 ºC). 28)

- This is the 10th confirmed planet discovered by TESS, and hundreds of additional candidates are now being studied.

- Scientists at MIT (Massachusetts Institute of Technology) and the Carnegie Institution for Science analyzed TESS transit data from the first four sectors of TESS observations to detect 11 periodic dips in the star’s brightness. From this, they determined that the star’s light was being partially blocked by a planet about the size of Earth.

- The star that HD 21749c orbits is bright and relatively nearby, and therefore well suited to more detailed follow-up studies, which could provide critical information about the planet’s properties, including potentially the first mass measurement of an Earth-size planet found by TESS. 29)

• March 26, 2019: NASA's new TESS (Transiting Exoplanet Survey Satellite) is designed to ferret out habitable exoplanets, but with hundreds of thousands of sunlike and smaller stars in its camera views, which of those stars could host planets like our own? A team of astronomers has identified the most promising targets for this search. 30) 31)

- TESS will observe 400,000 stars across the whole sky to catch a glimpse of a planet transiting across the face of its star, one of the primary methods by which exoplanets are identified.

- A team of astronomers from Cornell University, Lehigh University and Vanderbilt University has identified the most promising targets for this search in the new "TESS Habitable Zone Star Catalog," published in Astrophysical Journal Letters. Lead author is Lisa Kaltenegger, professor of astronomy at Cornell, director of Cornell's Carl Sagan Institute and a member of the TESS science team. 32)

- The catalog identifies 1,822 stars for which TESS is sensitive enough to spot Earth-like planets just a bit larger than Earth that receive radiation from their star equivalent to what Earth receives from our sun. For 408 stars, TESS can glimpse a planet just as small as Earth, with similar irradiation, in one transit alone.

- "Life could exist on all sorts of worlds, but the kind we know can support life is our own, so it makes sense to first look for Earth-like planets," Kaltenegger said. "This catalog is important for TESS because anyone working with the data wants to know around which stars we can find the closest Earth-analogs."

- Kaltenegger leads a program on TESS that is observing the catalog's 1,822 stars in detail, looking for planets. "I have 408 new favorite stars," said Kaltenegger. "It is amazing that I don't have to pick just one; I now get to search hundreds of stars."

- Confirming an exoplanet has been observed and figuring out the distance between it and its star requires detecting two transits across the star. The 1,822 stars the researchers have identified in the catalog are ones from which TESS could detect two planetary transits during its mission. Those orbital periods place them squarely in the habitable zone of their star.

- The habitable zone is the area around a star at which water can be liquid on a rocky planet's surface, therefore considered ideal for sustaining life. As the researchers note, planets outside the habitable zone could certainly harbor life, but it would be extremely difficult to detect any signs of life on such frozen planets without flying there.

- The catalog also identifies a subset of 227 stars for which TESS can not only probe for planets that receive the same irradiation as Earth, but for which TESS can also probe out farther, covering the full extent of the habitable zone all the way to cooler Mars-like orbits. This will allow astronomers to probe the diversity of potentially habitable worlds around hundreds of cool stars during the TESS mission's lifetime.

- The stars selected for the catalog are bright, cool dwarfs, with temperatures roughly between 2,700 and 5,000 degrees Kelvin. The stars in the catalog are selected due to their brightness; the closest are only approximately 6 light-years from Earth.

- "We don't know how many planets TESS will find around the hundreds of stars in our catalog or whether they will be habitable," Kaltenegger said, "but the odds are in our favor. Some studies indicate that there are many rocky planets in the habitable zone of cool stars, like the ones in our catalog. We're excited to see what worlds we'll find."

- A total of 137 stars in the catalog are within the continuous viewing zone of NASA's James Webb Space Telescope, now under construction. Webb will be able to observe them to characterize planetary atmospheres and search for signs of life in their atmospheres.

- Planets TESS identifies may also make excellent targets for observations by ground-based extremely large telescopes currently being built, the researchers note, as the brightness of their host stars would make them easier to characterize.

- "This is a remarkable time in human history and a huge leap for our understanding of our place in the universe," said Stassun, a member of the TESS science team.

• January 7, 2019: NASA's TESS mission has discovered a third small planet outside our solar system, scientists announced this week at the annual AAS (American Astronomical Society) meeting in Seattle, WA. 33) 34) 35)

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Figure 19: NASA’s TESS mission, which will survey the entire sky over the next two years, has already discovered three new exoplanets around nearby stars (image credit: NASA/GSFC, edited by MIT News)

- The new planet, named HD 21749b, orbits a bright, nearby dwarf star about 53 light years away, in the constellation Reticulum, and appears to have the longest orbital period of the three planets so far identified by TESS. HD 21749b journeys around its star in a relatively leisurely 36 days, compared to the two other planets — Pi Mensae b, a “super-Earth” with a 6.3-day orbit, and LHS 3844b, a rocky world that speeds around its star in just 11 hours. All three planets were discovered in the first three months of TESS observations.

- The surface of the new planet is likely around 300 degrees Fahrenheit (150ºC) — relatively cool, given its proximity to its star, which is almost as bright as the sun.

- “It’s the coolest small planet that we know of around a star this bright,” says Diana Dragomir, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, who led the new discovery. “We know a lot about atmospheres of hot planets, but because it’s very hard to find small planets that orbit farther from their stars, and are therefore cooler, we haven’t been able to learn much about these smaller, cooler planets. But here we were lucky, and caught this one, and can now study it in more detail.”

- The planet is about three times the size of Earth, which puts it in the category of a “sub-Neptune.” Surprisingly, it is also a whopping 23 times as massive as the Earth. But it is unlikely that the planet is rocky and therefore habitable; it’s more likely made of gas, of a kind that is much more dense than the atmospheres of either Neptune or Uranus.

- “We think this planet wouldn’t be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy,” Dragomir says. “The planet likely has a density of water, or a thick atmosphere.”

- Serendipitously, the researchers have also detected evidence of a second planet, though not yet confirmed, in the same planetary system, with a shorter, 7.8-day orbit. If it is confirmed as a planet, it could be the first Earth-sized planet discovered by TESS.

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Figure 20: NASA’s Transiting Exoplanet Survey Satellite (TESS) has found three confirmed exoplanets in the data from the space telescope’s four cameras (image credit: NASA/MIT/TESS, Ref. 35)

Figure 21: TESS first planet locations (video credit: NASA, MIT)

"Something there"

- Since TESS launched in April 2018, the spacecraft has been monitoring the sky, sector by sector, for momentary dips in the light of about 200,000 nearby stars. Such dips likely represent a planet passing in front of that star.

- The satellite trains its four onboard cameras on each sector for 27 days, taking in light from the stars in that particular segment before shifting to view the next one. Over its two-year mission, TESS will survey nearly the entire sky by monitoring and piecing together overlapping slices of the night sky. The satellite will spend the first year surveying the sky in the Southern Hemisphere, before swiveling around to take in the Northern Hemisphere sky.

- The mission has released to the public all the data TESS has collected so far from the first three of the 13 sectors that it will monitor in the southern sky. For their new analysis, the researchers looked through this data, collected between July 25 and Oct. 14.

- Within the sector 1 data, Dragomir identified a single transit, or dip, in the light from the star HD 21749. As the satellite only collects data from a sector for 27 days, it’s difficult to identify planets with orbits longer than that time period; by the time a planet passes around again, the satellite may have shifted to view another slice of the sky.

- To complicate matters, the star itself is relatively active, and Dragomir wasn’t sure if the single transit she spotted was a result of a passing planet or a blip in stellar activity. So she consulted a second dataset, collected by the HARPS (High Accuracy Radial velocity Planet Searcher), a high-precision spectrograph installed on a large ground-based telescope in Chile, which identifies exoplanets by their gravitational tug on their host stars.

- “They had looked at this star system a decade ago and never announced anything because they weren’t sure if they were looking at a planet versus the activity of the star,” Dragomir says. “But we had this one transit, and knew something was there.”

Stellar detectives

- When the researchers looked through the HARPS data, they discovered a repeating signal emanating from HD 21749 every 36 days. From this, they estimated that, if they indeed had seen a transit in the TESS data from sector 1, then another transit should appear 36 days later, in data from sector 3. When that data became publicly available, a momentary glitch created a gap in the data just at the time when Dragomir expected the second transit to occur.

- “Because there was an interruption in data around that time, we initially didn’t see a second transit, and were pretty disappointed,” Dragomir recalls. “But we re-extracted the data and zoomed in to look more carefully, and found what looked like the end of a transit.”

- She and her colleagues compared the pattern to the first full transit they had originally discovered, and found a near perfect match — an indication that the planet passed again in front of its star, in a 36-day orbit.

- “There was quite some detective work involved, and the right people were there at the right time,” Dragomir says. “But we were lucky and we caught the signals, and they were really clear.”

- They also used data from the Planet Finder Spectrograph, an instrument installed on the Magellan Telescope in Chile, to further validate their findings and constrain the planet’s mass and orbit.

- Once TESS has completed its two-year monitoring of the entire sky, the science team has committed to delivering information on 50 small planets less than four times the size of Earth to the astronomy community for further follow-up, either with ground-based telescopes or the future James Webb Space Telescope.

- “We’ve confirmed three planets so far, and there are so many more that are just waiting for telescope and people time to be confirmed,” Dragomir says. “So it’s going really well, and TESS is already helping us to learn about the diversity of these small planets.”

- TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by Goddard. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission.

• On 17 September 2018, TESS (Transiting Exoplanet Survey Satellite) shared its first science observations. Part of the data from TESS’s initial science orbit includes a detailed picture of the southern sky taken with all four of the planet-hunter’s wide-field cameras. The image captures a wealth of stars and other objects, including systems previously known to have exoplanets, planets beyond our solar system. TESS will spend the next two years monitoring the nearest, brightest stars for periodic dips in their brightness, known as transits. Such transits suggest a planet may be passing in front of its parent star. TESS is expected to find thousands of new planets using this method. 36) 37)

- TESS’s cameras, designed and built by MIT’s Lincoln Laboratory in Lexington, Massachusetts, and the MIT Kavli Institute, monitor large swaths of the sky to look for transits. Transits occur when a planet passes in front of its star as viewed from the satellite’s perspective, causing a regular dip in the star’s brightness.

- TESS will spend two years monitoring 26 such sectors for 27 days each, covering 85 percent of the sky. During its first year of operations, the satellite will study the 13 sectors making up the southern sky. Then TESS will turn to the 13 sectors of the northern sky to carry out a second year-long survey.

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Figure 22: TESS took this snapshot of the Large Magellanic Cloud (right) and the bright star R Doradus (left) with just a single detector of one of its cameras on 7 Aug. 2018. The frame is part of a swath of the southern sky TESS captured in its “first light” science image as part of its initial round of data collection (image credit: NASA/MIT/TESS)

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Figure 23: TESS captured this strip of stars and galaxies in the southern sky during one 30-minute period on 7 Aug. 2018. Created by combining the view from all four of its cameras, this is TESS’s “first light,” from the first observing sector that will be used for identifying planets around other stars. Notable features in this swath of the southern sky include the Large and Small Magellanic Clouds and a globular cluster called NGC 104, also known as 47 Tucanae. The brightest stars in the image, Beta Gruis and R Doradus, saturated an entire column of camera detector pixels on the satellite’s second and fourth cameras (image credit: NASA/MIT/TESS)

Figure 24: How NASA’s newest planet hunter scans the sky. This animation shows how TESS will study 85 percent of the sky in 26 sectors. The spacecraft will observe the 13 sectors that make up the southern sky in the first year and the 13 sectors of the northern sky in the second year (video credit: NASA/GSFC)

• December 6, 2018: The first batch of TESS mission data is now available through MAST. This release includes all data from Sectors 1 and 2, observed between July 25 and September 20, 2018. This includes both FFI (Full Frame Images) and 2-min cadence data. 38)

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Figure 25: Map of observations (image credit: STScI)

• August 6, 2018: Before NASA’s TESS started science operations on July 25, 2018, the planet hunter sent back a stunning sequence of serendipitous images showing the motion of a comet. Taken over the course of 17 hours on July 25, these TESS images helped demonstrate the satellite’s ability to collect a prolonged set of stable periodic images covering a broad region of the sky — all critical factors in finding transiting planets orbiting nearby stars. 39)

- Over the course of these tests, TESS took images of C/2018 N1, a comet discovered by NASA’s NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer) satellite on June 29. The comet, located about 29 million miles (48 million km) from Earth in the southern constellation Piscis Austrinus, is seen to move across the frame from right to left as it orbits the Sun. The comet’s tail, which consists of gases carried away from the comet by an outflow from the Sun called the solar wind, extends to the top of the frame and gradually pivots as the comet glides across the field of view.

Figure 26: The animated gif sequence is compiled from a series of images taken on July 25 by TESS. The angular extent of the widest field of view is six degrees. Visible in the images are the comet C/2018 N1, asteroids, variable stars, asteroids and reflected light from Mars. TESS is expected to find thousands of planets around other nearby stars (image credit: MIT, NASA/GSFC)

- In addition to the comet, the images reveal a treasure trove of other astronomical activity. The stars appear to shift between white and black as a result of image processing. The shift also highlights variable stars — which change brightness either as a result of pulsation, rapid rotation, or by eclipsing binary neighbors. Asteroids in our solar system appear as small white dots moving across the field of view. Towards the end of the video, one can see a faint broad arc of light moving across the middle section of the frame from left to right. This is stray light from Mars, which is located outside the frame. The images were taken when Mars was at its brightest near opposition, or its closest distance, to Earth.

- These images were taken during a short period near the end of the mission’s commissioning phase, prior to the start of science operations. The movie presents just a small fraction of TESS’s active field of view. The team continues to fine-tune the spacecraft’s performance as it searches for distant worlds.

• July 27, 2018: NASA’s TESS (Transiting Exoplanet Survey Satellite) has started its search for planets around nearby stars, officially beginning science operations on July 25, 2018. TESS is expected to transmit its first series of science data back to Earth in August, and thereafter periodically every 13.5 days, once per orbit, as the spacecraft makes it closest approach to Earth. The TESS Science Team will begin searching the data for new planets immediately after the first series arrives. 40)

- “I’m thrilled that our new planet hunter mission is ready to start scouring our solar system’s neighborhood for new worlds,” said Paul Hertz, NASA Astrophysics division director at Headquarters, Washington. “Now that we know there are more planets than stars in our universe, I look forward to the strange, fantastic worlds we’re bound to discover.”

- TESS is NASA’s latest satellite to search for planets outside our solar system, known as exoplanets. The mission will spend the next two years monitoring the nearest and brightest stars for periodic dips in their light. These events, called transits, suggest that a planet may be passing in front of its star. TESS is expected to find thousands of planets using this method, some of which could potentially support life.

• July 11, 2018: After a successful launch on April 18, 2018, NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite, is currently undergoing a series of commissioning tests before it begins searching for planets. The TESS team has reported that the spacecraft and cameras are in good health, and the spacecraft has successfully reached its final science orbit. The team continues to conduct tests in order to optimize spacecraft performance with a goal of beginning science at the end of July. 41)

- Every new mission goes through a commissioning period of testing and adjustments before beginning science operations. This serves to test how the spacecraft and its instruments are performing and determines whether any changes need to be made before the mission starts observations.

• May 21, 2018: TESS successfully completed a lunar flyby on 17 May at 06:34:35 UTC (2:34 AM EST). At its closest approach, TESS was 8,253 km from the lunar surface. Based on the successful lunar fly-by, no adjustment burn was required. 42)

TESS_Auto8

Figure 27: An artist’s illustration of TESS as it passed the Moon during its lunar flyby. This provided a gravitational boost that placed TESS on course for its final working orbit (image credit: NASA's Goddard Space Flight Center)

- As part of commissioning, the TESS science team took a 2-second test exposure using one of four TESS cameras, providing an exciting glimpse of the type of image expected from each of TESS’ four cameras. The image is centered on the southern constellation Centaurus with the bright star Beta Centauri is visible at the lower left edge. 43)

- TESS will undergo one final thruster burn on May 30 to enter its science orbit around Earth. This highly elliptical orbit will maximize the amount of sky the spacecraft can image, allowing it to continuously monitor large swaths of the sky. TESS is expected to begin science operations in mid-June after reaching this orbit and completing camera calibrations.

TESS_Auto7

Figure 28: This test image from one of the four cameras aboard TESS captures a swath of the southern sky along the plane of our galaxy. More than 200,000 stars are visible in this image. TESS is expected to cover more than 400 times the amount of sky shown in this image when using all four of its cameras during science operations. The image, which is centered in the constellation Centaurus, includes dark tendrils from the Coal Sack Nebula and the bright emission nebula Ced 122 (upper right).The bright star at bottom center is Beta Centauri (image credit: NASA/MIT/TESS)




Sensor complement: (Four WFOV cameras)

The TESS payload consists of four identical cameras and a DHU (Data Handling Unit). Each camera consists of a lens assembly with seven optical elements, and a detector assembly with four CCDs (Charge Coupled Devices) and their associated electronics. All four cameras are mounted onto a single plate (Figure 29) that is attached to the spacecraft, such that their FOVs are lined up to form a rectangle measuring 24º x 96º on the sky. Four elliptical holes in the plate allow shimless alignment of the four cameras at the desired angles. 44) 45) 46)

Each of the four cameras features:

- WFOV (Wide Field of View) of 24º x 24º

- 100 mm effective pupil diameter

- Lens assembly with 7 optical elements

- Athermal design

- 600nm - 1000 nm bandpass

- 16.8 Mpixel, low-noise, low-power, MIT/LL CCID-80 detector.

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Figure 29: Illustration of the four cameras mounted on a single plate (image credit: NASA, MIT)


Detector assembly: The focal plane consists of four back-illuminated MIT/LL CCID-80 devices. The CCID-80, developed for TESS, is a deep-depletion, frame-transfer CCD with a full frame store. The device has four outputs; each output is associated with an array of 512 (H) x 2048 (V) imaging pixels, for a total imaging area of 2048 (H) x 2048 (V). The die size is 32 (W) x 64 (H) mm for an area of 20.4 cm2. 47)

The imaging array, frame store, and serial registers all consist of conventional three-phase, 15 x 15 µm pixels. There is a three-phase charge injection register at the top of the array, and the serial register support bidirectional transfer. The pixel array employs a trough design feature to provide radiation mitigation for small charge packets. To enable the desired fast frame transfer time, the image array and frame store clocks are strapped with metal interconnect to reduce the RC delay from the clock lines. The output circuit is a single-stage MOSFET similar to others demonstrated at Lincoln Laboratory.

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Figure 30: The detector assembly of one of the prototype lenses. The light shield cover for the frame store regions is removed (image credit: MIT/LL)

Lincoln Laboratory supports several different styles of back-illumination processing. For TESS, a flow is used that involves: epoxy mounting the device wafer to a support wafer; wet chemical thinning the high resistivity float zone silicon to the 100 µm full depletion target; back-side passivation through an ion implantation, laser annealing sequence; deposition and patterning of antireflection and light shield coatings; and etches to provide access to the bond pads.

The true benefit of the 100 µm thick detector is shown in the Figure 31 spectral response curve. The project observed over 20% improvement in quantum efficiency at 1000 nm measurement wavelength over a 45 µm thick device. - A total of sixteen CCDs arranged in four mosaics will be needed.

TESS_Auto4

Figure 31: Measured quantum efficiency for the 100 µm thick CCID-80 device compared to a 45 µm thick reference MIT/LL CCD (image credit: MIT/LL)

Each of the four identical TESS lenses is an f=1:4 custom design consisting of seven optical elements, with an entrance pupil diameter of 10.5 cm (Figures 32 and 33). For ease of manufacture, all lens surfaces are spherical except for two mild aspheres. There are two separate aluminum lens barrels that are fastened and pinned together. All optical elements have antireflection coatings. The surface of one element also has a long-pass filter coating to enforce the cutoff at 600 nm. The red limit at 1000 nm is set by the quantum-efficiency curve of the CCDs (Figure 31).

Each lens forms a 24º x 24º unvignetted image on the four-CCD mosaic in its focal plane. The optical design was optimized to provide small image spots of a consistent size across the FOV (Field of View), and produce undersampled images similar to those of Kepler. At nominal focus and flight temperature (-75ºC), the 50% ensquared-energy half-width is 15 µm (one pixel or 0.35 arcmin) averaged over the FOV. Each lens is equipped with a lens hood, which reduces the effects of scattered light from the Earth and Moon (Ref. 3).

FOV (Field of View) of each lens

24º x 24º

Combined field of view

24º x 96º = 2300ºº (sq. deg.)

Entrance pupil diameter

10.5 cm

Focal ratio (f=#)

f/1.4

Wavelength range

600 - 1000 nm

Ensquared energy

50% within 15 x 15 µm (one pixel, or 0:35 x 0:35 arcmin)
90% within 60 x 60 µm (4 x 4 pixels, or 1:4 x 1:4 arcmin)

Table 3: Characteristics of the TESS lenses. Ensquared energy is the fraction of the total energy of the point-spread function that is within a square of the given dimensions centered on the peak.

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Figure 32: Diagram of the TESS lens assembly, CCD focal plane, and detector electronics (image credit: NASA, TESS Team)

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Figure 33: Left: Two lens prototypes that were constructed. Right: The detector assembly of one of the prototype lenses. The frame-store regions of the CCDs are covered (image credit: NASA, TESS Team)


Scanning strategy: The four cameras act as a 1 x 4 array, providing a combined FOV of 24º x 96º or 2300 square degrees (Figure 34). The north and south ecliptic hemispheres are each divided into 13 partially overlapping sectors of 24º x 96º, extending from an ecliptic latitude of 6º to the ecliptic pole. Each sector is observed continuously for two spacecraft orbits (27.4 days), with the boresight of the four-camera array pointed nearly anti-solar. After two orbits, the FOV is shifted eastward in ecliptic longitude by about 27º, to observe the next sector. Observing an entire hemisphere takes one year, and the all-sky survey takes two years.

The overlap of the sectors is illustrated in Figure 34. Approximately 30,000 square degrees are observed for at least 27 days. Close to the ecliptic poles, approximately 2800 square degrees are observed for more than 80 days. Surrounding the ecliptic poles, approximately 900 square degrees are observed for more than 300 days.

TESS_Auto1

Figure 34: Left: The instantaneous combined FOV of the four TESS cameras. Middle: Division of the celestial sphere into 26 observation sectors (13 per hemisphere). Right: Duration of observations on the celestial sphere, taking into account the overlap between sectors. The dashed black circle enclosing the ecliptic pole shows the region which JWST will be able to observe at any time (image credit: TESS Team)


Photometric performance: Figure 35 shows the anticipated photometric performance of the TESS cameras. The noise sources in this model are photon-counting noise from the star and the background (zodiacal light and faint unresolved stars), dark current (negligible), readout noise, and a term representing additional systematic errors that cannot be corrected by co-trending. The most important systematic error is expected to be due to random pointing variations (”spacecraft jitter"). Because of the non-uniform quantum efficiency of the CCD pixels, motion of the star image on the CCD will introduce changes in the measured brightness, as the weighting of the image PSF (Point Spread Function) changes, and as parts of the image PSF enter and exit the summed array of pixels.

The central pixel of a stellar image will saturate at approximately IC = 7:5. However, this does not represent the bright limit for precise photometry because the excess charge is spread across other CCD pixels and is conserved, until the excess charge reaches the boundary of the CCD. As long as the photometric aperture is large enough to encompass all of the charge, high photometric precision can still be obtained. The Kepler mission demonstrated that photon-noise{limited photometry can be obtained for stars 4 mag brighter than the single-pixel saturation limit. Since similar performance is expected for TESS, the bright limit is expected to be IC~4 or perhaps even brighter.

TESS_Auto0

Figure 35: Top: Expected 1σ photometric precision as a function of stellar apparent magnitude in the IC band. Contributions are from photon-counting noise from the target star and background (zodiacal light and unresolved stars), detector read noise (10 e-), and an assumed 60 ppm of incorrigible noise on hourly timescales. Bottom: The number of pixels in the photometric aperture that optimizes the signal-to-noise ratio (image credit: TESS Team)



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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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