Minimize TESS (Transiting Exoplanet Survey Satellite)

TESS (Transiting Exoplanet Survey Satellite)

Spacecraft    Launch    Mission Status    Sensor Complement    References 

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

• January 20, 2022: The catalog of planet candidates found with NASA’s Transiting Exoplanet Survey Satellite (TESS) recently passed 5,000 TOIs (TESS Objects of Interest). - Catalog of planet candidates nearly doubles in size during 2020-21. 15)

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Figure 9: A map of the sky is now crowded with over 5,000 exoplanet candidates from NASA’s TESS mission. The TESS Science Office at MIT released the most recent batch of TESS Objects of Interest (large orange points on the map) on Dec. 21, boosting the catalog to this 5,000-count milestone (Credits: Image courtesy of NASA/MIT/TESS)

- The catalog has been growing steadily since the start of the mission in 2018, and the batch of TOIs boosting the catalog to over 5,000 come mostly from the Faint Star Search led by MIT postdoc Michelle Kunimoto.

- Kunimoto reflects, “This time last year, TESS had found just over 2,400 TOIs. Today, TESS has reached more than twice that number — a huge testament to the mission and all the teams scouring the data for new planets. I’m excited to see thousands more in the years to come!”

- Now in its extended mission, TESS is observing the Northern Hemisphere and ecliptic plane, including regions of the sky previously observed by the Kepler and K2 missions. The TOIs added in late December are from the third year of the TESS mission, which ran from July 2020 to June 2021. TESS re-observed the sky visible in the Earth’s Southern Hemisphere, revisiting stars it had first observed at the mission’s start in 2018.

- TOI manager Katharine Hesse remarks, “With data from the first year of the extended mission, we have found dozens of additional candidates to TOIs found during the prime mission. I am excited to see how many multi-planet systems we can find during the rest of the extended mission and in upcoming years with TESS.” Planned extensions of the TESS mission to 2025 and beyond should unveil many more new planet candidates.

- Discovering more planet candidates and adding them to the TESS Objects of Interest Catalog is the first step. In the coming months, astronomers around the world will study each of these TOIs to confirm whether they are bona fide planets, and the catalog of confirmed exoplanets from the TESS mission (175 as of Dec. 20) will continue to grow.

- 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 Center for Astrophysics | Harvard and Smithsonian 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.

• January 13, 2022: Three newly-discovered planets have been orbiting dangerously close to stars nearing the end of their lives. 16)

- Out of the thousands of extrasolar planets found so far, these three gas giant planets first detected by the NASA TESS (Transiting Exoplanet Survey Satellite) Mission, have some of the shortest-period orbits around subgiant or giant stars. One of the planets, TOI-2337b, will be consumed by its host star in less than 1 million years, sooner than any other currently known planet.

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Figure 10: An artist’s rendition of what a planetary system similar to TOI-2337b, TOI-4329b, and TOI-2669b might look like, where a hot Jupiter-like exoplanet orbits an evolved, dying star (image credit: Karen Teramura/University of Hawai'i Institute for Astronomy)

- “These discoveries are crucial to understanding a new frontier in exoplanet studies: how planetary systems evolve over time,” explained lead author Samuel Grunblatt, a postdoctoral fellow at the American Museum of Natural History and the Flatiron Institute in New York City. Grunblatt, who earned his PhD from the University of Hawai'i Institute for Astronomy (UH IfA), added that “these observations offer new windows into planets nearing the end of their lives, before their host stars swallow them up.”

- Grunblatt announced the discovery and confirmation of these planets – TOI-2337b, TOI-4329b, and TOI-2669b – at an American Astronomical Society press conference today; the study has been accepted for publication in the Astronomical Journal and is available in preprint format on arXiv.org.

- The researchers estimate that the planets have masses between 0.5 and 1.7 times Jupiter’s mass, and sizes that range from slightly smaller to more than 1.6 times the size of Jupiter. They also span a wide range of densities, from styrofoam-like to three times denser than water, implying a wide variety of origins.

- These three planets are believed to be just the tip of the iceberg. “We expect to find tens to hundreds of these evolved transiting planet systems with TESS, providing new details on how planets interact with each other, inflate, and migrate around stars, including those like our Sun,” said Nick Saunders, a graduate student at UH IfA and co-author of the study.

- The planets were first found in NASA TESS Mission full-frame image data taken in 2018 and 2019. Grunblatt and his collaborators identified the candidate planets in TESS data, and then used W. M. Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES) on Maunakea, Hawai'i to confirm the existence of the three planets.

- “The Keck observations of these planetary systems are critical to understanding their origins, helping reveal the fate of solar systems like our own,” said UH IfA Astronomer Daniel Huber, who co-authored the study.

- Current models of planet dynamics suggest that planets should spiral in toward their host stars as the stars evolve over time, particularly in the last 10 percent of the star’s lifetime. This process also heats the planets, potentially causing their atmospheres to inflate. However, this stellar evolution will also cause the orbits of planets around the host star to come closer to one another, increasing the likelihood that some of them will collide, or even destabilize the entire planetary system.

- The wide variety of planet densities found in the study suggests that these planetary systems have been shaped through chaotic planet-to-planet interactions. This could also have resulted in unpredictable heating rates and timescales for these planets, giving them the wide range in densities we observe today.

- Future observations of one of these systems, TOI-4329, with the recently-launched James Webb Space Telescope could reveal evidence for water or carbon dioxide in the planet’s atmosphere. If these molecules are seen, the data would provide constraints on where these planets formed, and what sort of interactions had to occur to produce the planetary orbits we see today.

- Continued monitoring of these systems with the NASA TESS telescope will constrain the rate at which these planets are spiraling into their host stars. So far, no clear signal of orbital decay has been observed in any of the systems, but a longer baseline of observations with the TESS Extended Missions will provide much tighter constraints on planet in-spiral than are currently possible, revealing how strongly planetary systems are affected by stellar evolution.

- The team hopes that this ‘planetary archeology’ will help us to understand the past, present, and future of planetary systems, moving us one step closer to answering the question: “Are we alone?”

About HIRES

- The High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many “stripes” of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding exoplanets. Astronomers also use HIRES to study important astrophysical phenomena like distant galaxies and quasars, and find cosmological clues about the structure of the early universe, just after the Big Bang.

• December 2, 2021: As far as extrasolar planets go, 'GJ 367 b' is a featherweight. With half the mass of Earth, the newly discovered planet is one of the lightest among the nearly 5000 exoplanets known today. It takes the extrasolar planet approximately eight hours to orbit its parent star. With a diameter of just over 9000 km, GJ 367 b is slightly larger than Mars. The planetary system is located just under 31 light years from Earth and is thus ideal for further investigation. The discovery demonstrates that it is possible to precisely determine the properties of even the smallest, least massive exoplanets. Such studies provide a key to understanding how terrestrial planets form and evolve. 17)

- An international group of 78 researchers led by Kristine W. F. Lam and Szilárd Csizmadia from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Institute of Planetary Research report on the results of their studies in the scientific journal Science. With an orbital period of only one-third of an Earth day, GJ 367 b is a fast mover. "From the precise determination of its radius and mass, GJ 367b is classified as a rocky planet," reports Kristine Lam. "It seems to have similarities to Mercury. This places it among the sub-Earth sized terrestrial planets and brings research one step forward in the search for a 'second Earth'." 18)

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Figure 11: Artist's rendition of Planet GI 367 [image credit: SPP 1992 (Patricia Klein)]

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Figure 12: Mass and radius of known small planets [image credit: Science (Lam et al. 2021)]

More precise exoplanet trackers possible

- A quarter of a century after the first discovery of an extrasolar planet, the focus has shifted to characterizing these planets more precisely, in addition to making new discoveries. At present, it is possible to construct a much more precise profile for most known exoplanets. Many exoplanets were discovered using the transit method – the measurement of minute differences in the emitted light, or apparent magnitude, of a star as a planet passes in front of it (with respect to the observer). GJ 367 b was also discovered using this method, with the help of NASA's Transiting Exoplanet Survey Satellite (TESS).

Origin of the small fast-moving planets still unknown

- GJ 367 b belongs to the 'ultra-short period' (USP) group of exoplanets that orbit their star in less than 24 hours. "We already know a few of these, but their origins are currently unknown," says Kristine Lam. "By measuring the precise fundamental properties of the USP planet, we can get a glimpse of the system's formation and evolution history." Following the discovery of this planet using TESS and the transit method, the spectrum of its star was then studied from the ground using the radial velocity method. The mass was determined using the HARPS instrument on the European Southern Observatory's 3.6m Telescope. With the meticulous study and combination of different evaluation methods, the radius and mass of the planet were precisely determined: its radius is 72 percent that of Earth's, and its mass 55 percent that of Earth's.

Highest precision for radius and mass

- By determining its radius and mass with a precision of 7 and 14 percent respectively, the researchers were also able to draw conclusions about the exoplanet's inner structure. It is a low-mass rocky planet, but is more dense than the Earth. "The high density indicates the planet is dominated by an iron core," explains Szilárd Csizmadia. "These properties are similar to those of Mercury, with its disproportionately large iron and nickel core that differentiates it from other terrestrial bodies in the Solar System." However, the planet's proximity to its star means it is exposed to extremely high levels of radiation, more than 500 times stronger than what the Earth experiences. The surface temperature could reach up to 1500 degrees Celsius – a temperature at which all rocks and metals would be melted. Therefore, GJ 367 b cannot be considered a 'second Earth'.

Parent star is a 'red dwarf'

- The parent star of this newly discovered exoplanet, a red dwarf called GJ 367, is only about half the size of the Sun. This was beneficial for its discovery as the transit signal of the orbiting planet is particularly significant. Red dwarfs are not only smaller, but also cooler than the Sun. This makes their associated planets easier to find and characterize. They are among the most common stellar objects in our cosmic neighborhood and are therefore suitable targets for exoplanet research. Researchers estimate that these red dwarfs, also known as 'class M stars', are orbited by an average of two to three planets.

• August 27, 2021: Brown dwarfs are astronomical objects with masses between those of planets and stars. The question of where exactly the limits of their mass lie remains a matter of debate, especially since their constitution is very similar to that of low-mass stars. So how do we know whether we are dealing with a brown dwarf or a very low mass star? An international team, led by scientists from the University of Geneva (UNIGE) and the Swiss National Centre of Competence in Research (NCCR) PlanetS, in collaboration with the University of Bern, has identified five objects that have masses near the border separating stars and brown dwarfs that could help scientists understand the nature of these mysterious objects. The results can be found in the journal Astronomy & Astrophysics. 19) 20)

- Like Jupiter and other giant gas planets, stars are mainly made of hydrogen and helium. But unlike gas planets, stars are so massive and their gravitational force so powerful that hydrogen atoms fuse to produce helium, releasing huge amounts of energy and light.

‘Failed stars’

- Brown dwarfs, on the other hand, are not massive enough to fuse hydrogen and therefore cannot produce the enormous amount of light and heat of stars. Instead, they fuse relatively small stores of a heavier atomic version of hydrogen: deuterium. This process is less efficient and the light from brown dwarfs is much weaker than that from stars. This is why scientists often refer to them as ‘failed stars’.

- “However, we still do not know exactly where the mass limits of brown dwarfs lie, limits that allow them to be distinguished from low-mass stars that can burn hydrogen for many billions of years, whereas a brown dwarf will have a short burning stage and then a colder life” points out Nolan Grieves, a researcher in the Department of Astronomy at the UNIGE’s Faculty of Science, a member of the NCCR PlanetS and the study’s first author. “These limits vary depending on the chemical composition of the brown dwarf, for example, or the way it formed, as well as its initial radius,” he explains. To get a better idea of what these mysterious objects are, we need to study examples in detail. But it turns out that they are rather rare. “So far, we have only accurately characterized about 30 brown dwarfs,” says the Geneva-based researcher. Compared to the hundreds of planets that astronomers know in detail, this is very few. All the more so if one considers that their larger size makes brown dwarfs easier to detect than planets.

New pieces to the puzzle

- Today, the international team characterized five companions that were originally identified with the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOI) – TOI-148, TOI-587, TOI-681, TOI-746 and TOI-1213. These are called ‘companions’ because they orbit their respective host stars. They do so with periods of 5 to 27 days, have radii between 0.81 and 1.66 times that of Jupiter and are between 77 and 98 times more massive. This places them on the borderline between brown dwarfs and stars.

- These five new objects therefore contain valuable information. “Each new discovery reveals additional clues about the nature of brown dwarfs and gives us a better understanding of how they form and why they are so rare,” says Monika Lendl, a researcher in the Department of Astronomy at the UNIGE and a member of the NCCR PlanetS.

- One of the clues the scientists found to show these objects are brown dwarfs is the relationship between their size and age, as explained by François Bouchy, professor at UNIGE and member of the NCCR PlanetS: “Brown dwarfs are supposed to shrink over time as they burn up their deuterium reserves and cool down. Here we found that the two oldest objects, TOI 148 and 746, have a smaller radius, while the two younger companions have larger radii.”

- Yet these objects are so close to the limit that they could just as easily be very low-mass stars, and astronomers are still unsure whether they are brown dwarfs. “Even with these additional objects, we still lack the numbers to draw definitive conclusions about the differences between brown dwarfs and low-mass stars. Further studies are needed to find out more,” concludes Grieves.

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Figure 13: Artist’s illustration showing the five brown dwarfs (TOIs) as well as the Sun, Jupiter and a low mass star for reference (image credit: Thibaut Roger)

• August 6, 2021: The leaders of a NASA exoplanet mission are considering using a spare camera for a companion mission that would enable them to confirm existing discoveries and make new ones. 21)

- NASA's TESS launched in April 2018 to perform an all-sky survey. The spacecraft’s four cameras observe regions of the sky for weeks at a time, looking for minute dips in brightness of stars caused when exoplanets cross is front of, or transit, those stars.

- TESS, which completed its two-year primary mission in 2020 and is now in an extended mission, has discovered thousands of potential exoplanets. In an Aug. 2 talk at the second TESS Science Conference, George Ricker, principal investigator for TESS at the Massachusetts Institute of Technology, said the mission had so far discovered 4,349 “objects of interest,” of which 3,667 were planet candidates. About 800 of those planet candidates are small exoplanets, with radii no more than four times that of the Earth.

- The spacecraft remains in good condition. “The spacecraft and the instruments are all working really well,” said Roland Vanderspek, deputy principal investigator for TESS, in a separate presentation at the conference. “There’s really been no change in their performance since the beginning of the mission.”

- The project is gearing up for a second extended mission that would start in October 2022 and last for three years. The health of the spacecraft along with its stable orbit, a highly elliptical two-week orbit around the Earth, give project officials confidence that TESS could operate into the next decade.

- At the conference, Ricker discussed a proposal for a companion mission to TESS, currently known as TESS-L5. That would be a small spacecraft equipped with a camera that was built as a flight spare for the four on TESS. The spacecraft would operate from the Earth-sun L-5 Lagrange point, one astronomical unit, or about 150 million kilometers, from the Earth.

- TESS-L5 would be able to perform observations of the same field of view of any of the cameras on TESS. “There is nothing that inspires more confidence in the validity of an observation than making that same observation at the same time from two different locations using near-identical detectors and getting the same result,” Ricker said in an email.

- The long baseline between TESS, orbiting the Earth, and TESS-L5 could enable additional science, such as searching for solar system objects. “You will be able to utilize parallax measurements to determine where the objects observed actually are,” he said, estimating the joint observations could find 500 transneptunian objects in the outer solar system. TESS-L5 alone, from its vantage point, could also detect near Earth objects approaching the Earth from the direction of sun, which are difficult to otherwise observe.

- The TESS-L5 mission would use laser communications to transmit data back to Earth. That system would provide six megabits per second of bandwidth and would use one-meter telescopes on Earth to receive the signals. Those downlinks would take place in daylight, meaning that the telescopes could be for astronomical observations at night as well.

- Ricker said TESS-L5 is still a concept at a fairly early stage, although the project has studied the feasibility of the laser communications system and is looking into “significant private contribution to parts of the system,” such as the ground stations. It’s unclear how much the mission would cost, although as a smallsat using spare flight hardware its costs could be kept down.

- A decision about whether to pursue TESS-L5 will depend on the state of TESS itself. If TESS continues to function well and have its mission extended, “we could possibly launch TESS-L5 during the 2026-2029 timeframe,” he said.

• August 4, 2021: Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have identified an unprecedented collection of pulsating red giant stars all across the sky. These stars, whose rhythms arise from internal sound waves, provide the opening chords of a symphonic exploration of our galactic neighborhood. 22)

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Figure 14: Red giant stars near and far sweep across the sky in this illustration. Measurements from NASA’s Transiting Exoplanet Survey Satellite have identified more than 158,000 pulsating red giants across nearly the entire sky. Such discoveries hold great potential for exploring the detailed structure of our home galaxy [image credit: NASA’s Goddard Space Flight Center/Chris Smith (KBRwyle)]

- TESS primarily hunts for worlds beyond our solar system, also known as exoplanets. But its sensitive measurements of stellar brightness make TESS ideal for studying stellar oscillations, an area of research called asteroseismology.

- “Our initial result, using stellar measurements across TESS’s first two years, shows that we can determine the masses and sizes of these oscillating giants with precision that will only improve as TESS goes on,” said Marc Hon, a NASA Hubble Fellow at the University of Hawaii in Honolulu. “What’s really unparalleled here is that TESS’s broad coverage allows us to make these measurements uniformly across almost the entire sky.”

Figure 15: This visualization shows the new sample of oscillating red giant stars (colored dots) discovered by NASA’s Transiting Exoplanet Survey Satellite. The colors map to each 24-by-96-degree swath of the sky observed during the mission's first two years. The view then changes to show the positions of these stars within our galaxy, based on distances determined by ESA’s (the European Space Agency’s) Gaia mission. The scale shows distances in kiloparsecs, each equal to 3,260 light-years, and extends nearly 20,000 light-years from the Sun (video credit: Credit: Kristin Riebe, Leibniz Institute for Astrophysics Potsdam, Germany)

- Hon presented the research during the second TESS Science Conference, an event supported by the Massachusetts Institute of Technology in Cambridge – held virtually from Aug. 2 to 6 – where scientists discuss all aspects of the mission. The Astrophysical Journal has accepted a paper describing the findings, led by Hon.

- Sound waves traveling through any object – a guitar string, an organ pipe, or the interiors of Earth and the Sun – can reflect and interact, reinforcing some waves and canceling out others. This can result in orderly motion called standing waves, which create the tones in musical instruments.

- Just below the surfaces of stars like the Sun, hot gas rises, cools, and then sinks, where it heats up again, much like a pan of boiling water on a hot stove. This motion produces waves of changing pressure – sound waves – that interact, ultimately driving stable oscillations with periods of a few minutes that produce subtle brightness changes. For the Sun, these variations amount to a few parts per million. Giant stars with masses similar to the Sun’s pulsate much more slowly, and the corresponding brightness changes can be hundreds of times greater.

- Oscillations in the Sun were first observed in the 1960s. Solar-like oscillations were detected in thousands of stars by the French-led Convection, Rotation and planetary Transits (CoRoT) space telescope, which operated from 2006 to 2013. NASA’s Kepler and K2 missions, which surveyed the sky from 2009 to 2018, found tens of thousands of oscillating giants. Now TESS extends this number by another 10 times.

- “With a sample this large, giants that might occur only 1% of the time become pretty common,” said co-author Jamie Tayar, a Hubble Fellow at the University of Hawaii. “Now we can start thinking about finding even rarer examples.”

- The physical differences between a cello and a violin produce their distinctive voices. Similarly, the stellar oscillations astronomers observe depend on each star’s interior structure, mass, and size. This means asteroseismology can help determine fundamental properties for large numbers of stars with accuracies not achievable in any other way.

Figure 16: Listen to the rhythms of three red giants in the constellation Draco, as determined by brightness measurements from NASA’s Transiting Exoplanet Survey Satellite. To produce audible tones, astronomers multiplied the oscillation frequencies of the stars by 3 million times. It’s clear that larger stars produce longer, deeper pulsations than smaller ones (video credit: NASA/MIT/TESS and Ethan Kruse (USRA), M. Hon et al., 2021)

- When stars similar in mass to the Sun evolve into red giants, the penultimate phase of their stellar lives, their outer layers expand by 10 or more times. These vast gaseous envelopes pulsate with longer periods and larger amplitudes, which means their oscillations can be observed in fainter and more numerous stars.

- TESS monitors large swaths of the sky for about a month at a time using its four cameras. During its two-year primary mission, TESS covered about 75% of the sky, each camera capturing a full image measuring 24-by-24 degrees every 30 minutes. In mid-2020, the cameras began collecting these images at an even faster pace, every 10 minutes.

- The images were used to develop light curves – graphs of changing brightness – for nearly 24 million stars over 27 days, the length of time TESS stares at each swath of the sky. To sift through this immense accumulation of measurements, Hon and his colleagues taught a computer to recognize pulsating giants. The team used machine learning, a form of artificial intelligence that trains computers to make decisions based on general patterns without explicitly programming them.

Figure 17: NASA’s TESS imaged about 75% of the sky during its two-year-long primary mission. This plot dissolves between the TESS sky map and a “mass map” constructed by combining TESS measurements of 158,000 oscillating red giant stars with their distances, established by ESA’s (the European Space Agency’s) Gaia mission. The prominent band in both images is the Milky Way, which marks the central plane of our galaxy. In the mass map, green, yellow, orange, and red show where giant stars average more than 1.4 times the Sun’s mass. Such stars evolve faster than the Sun, becoming giants at younger ages. The close correspondence of higher-mass giants with the plane of the Milky Way, which contains our galaxy's spiral arms, demonstrates that it contains many young stars (image credit: NASA/MIT/TESS and Ethan Kruse (USRA), M. Hon et al., 2021)

- To train the system, the team used Kepler light curves for more than 150,000 stars, of which some 20,000 were oscillating red giants. When the neural network finished processing all of the TESS data, it had identified a chorus of 158,505 pulsating giants.

- Next, the team found distances for each giant using data from ESA’s (the European Space Agency’s) Gaia mission, and plotted the masses of these stars across the sky. Stars more massive than the Sun evolve faster, becoming giants at younger ages. A fundamental prediction in galactic astronomy is that younger, higher-mass stars should lie closer to the plane of the galaxy, which is marked by the high density of stars that create the glowing band of the Milky Way in the night sky.

- “Our map demonstrates for the first time empirically that this is indeed the case across nearly the whole sky,” said co-author Daniel Huber, an assistant professor for astronomy at the University of Hawaii. “With the help of Gaia, TESS has now given us tickets to a red giant concert in the sky.”

• July 12, 2021: Thanks to data from NASA’s Transiting Exoplanet Survey Satellite (TESS), an international collaboration of astronomers has identified four exoplanets, worlds beyond our solar system, orbiting a pair of related young stars called TOI 2076 and TOI 1807. 23)

- These worlds may provide scientists with a glimpse of a little-understood stage of planetary evolution.

- “The planets in both systems are in a transitional, or teenage, phase of their life cycle,” said Christina Hedges, an astronomer at the Bay Area Environmental Research Institute in Moffett Field and NASA’s Ames Research Center in Silicon Valley, both in California. “They’re not newborns, but they’re also not settled down. Learning more about planets in this teen stage will ultimately help us understand older planets in other systems.”

- A paper describing the findings, led by Hedges, was published in The Astronomical Journal.

Figure 18: Stellar siblings over 130 light-years away host two systems of teenage planets. Watch to learn how NASA’s Transiting Exoplanet Survey Satellite discovered these young worlds and what they might tell us about the evolution of planetary systems everywhere, including our own [video credits: NASA’s Goddard Space Flight Center/Chris Smith (KBRwyle)]

- TOI 2076 and TOI 1807 reside over 130 light-years away with some 30 light-years between them, which places the stars in the northern constellations of Boötes and Canes Venatici, respectively. Both are K-type stars, dwarf stars more orange than our Sun, and around 200 million years old, or less than 5% of the Sun’s age. In 2017, using data from ESA’s (the European Space Agency’s) Gaia satellite, scientists showed that the stars are traveling through space in the same direction.

- Astronomers think the stars are too far apart to be orbiting each other, but their shared motion suggests they are related, born from the same cloud of gas.

- Both TOI 2076 and TOI 1807 experience stellar flares that are much more energetic and occur much more frequently than those produced by our own Sun.

- “The stars produce perhaps 10 times more UV light than they will when they reach the Sun’s age,” said co-author George Zhou, an astrophysicist at the University of Southern Queensland in Australia. “Since the Sun may have been equally as active at one time, these two systems could provide us with a window into the early conditions of the solar system.”

- TESS monitors large swaths of the sky for nearly a month at a time. This long gaze allows the satellite to find exoplanets by measuring small dips in stellar brightness caused when a planet crosses in front of, or transits, its star.

- Alex Hughes initially brought TOI 2076 to astronomers’ attention after spotting a transit in the TESS data while working on an undergraduate project at Loughborough University in England, and he has since graduated with a bachelor’s degree in physics. Hedges’ team eventually discovered three mini-Neptunes, worlds between the diameters of Earth and Neptune, orbiting the star. Innermost planet TOI 2076 b is about three times Earth’s size and circles its star every 10 days. Outer worlds TOI 2076 c and d are both a little over four times larger than Earth, with orbits exceeding 17 days.

- TOI 1807 hosts only one known planet, TOI 1807 b, which is about twice Earth’s size and orbits the star in just 13 hours. Exoplanets with such short orbits are rare. TOI 1807 b is the youngest example yet discovered of one of these so-called ultra-short period planets.

- Scientists are currently working to measure the planets’ masses, but interference from the hyperactive young stars could make this challenging.

- According to theoretical models, planets initially have thick atmospheres left over from their formation in disks of gas and dust around infant stars. In some cases, planets lose their initial atmospheres due to stellar radiation, leaving behind rocky cores. Some of those worlds go on to develop secondary atmospheres through planetary processes like volcanic activity.

- The ages of the TOI 2076 and TOI 1807 systems suggest that their planets may be somewhere in the middle of this atmospheric evolution. TOI 2076 b receives 400 times more UV light from its star than Earth does from the Sun – and TOI 1807 b gets around 22,000 times more.

- If scientists can discover the planets’ masses, the information could help them determine if missions like NASA’s Hubble and upcoming James Webb space telescopes can study the planets’ atmospheres – if they have them.

- The team is particularly interested in TOI 1807 b because it’s an ultra-short period planet. Theoretical models suggest it should be difficult for worlds to form so close to their stars, but they can form farther out and then migrate inward. Because it would have had to both form and migrate in just 200 million years, TOI 1807 b will help scientists further understand the life cycles of these types of planets. If it doesn’t have a very thick atmosphere and its mass is mostly rock, the planet’s proximity to its star could potentially mean its surface is covered in oceans or lakes of molten lava.

- “Many objects we study in astronomy evolve on such long timescales that a human being can’t see changes month to month or year to year,” said co-author Trevor David, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics in New York. “If you want to see how planets evolve, your best bet is to find many planets of different ages and then ask how they’re different. The TESS discovery of the TOI 2076 and TOI 1807 systems advances our understanding of the teenage exoplanet stage.”

• April 30, 2021: NASA has a long tradition of unexpected discoveries, and the space program's TESS mission is no different. SMU astrophysicist and her team have discovered a particularly bright gamma-ray burst using a NASA telescope designed to find exoplanets — those occurring outside our solar system — particularly those that might be able to support life. 24)

- It's the first time a gamma-ray burst has been found this way.

- Gamma-ray bursts are the brightest explosions in the universe, typically associated with the collapse of a massive star and the birth of a black hole. They can produce as much radioactive energy as the sun will release during its entire 10-billion-year existence.

- Krista Lynne Smith, an assistant professor of physics at Southern Methodist University, and her team confirmed the blast — called GRB 191016A — happened on Oct. 16 and also determined its location and duration. A study on the discovery has been published in The Astrophysical Journal. 25)

- "Our findings prove this TESS telescope is useful not just for finding new planets, but also for high-energy astrophysics," said Smith, who specializes in using satellites like TESS (Transiting Exoplanet Survey Satellite) to study supermassive black holes and gas that surrounds them. Such studies shed light on the behavior of matter in the deeply warped spacetime around black holes and the processes by which black holes emit powerful jets into their host galaxies.

- Smith calculated that GRB 191016A had a peak magnitude of 15.1, which means it was 10,000 times fainter than the faintest stars we can see with the naked eyes.

- That may sound quite dim, but the faintness has to do with how far away the burst occurred. It is estimated that light from GRB 191016A's galaxy had been travelling 11.7 billion years before becoming visible in the TESS telescope.

- Most gamma ray bursts are dimmer – closer to 160,000 times fainter than the faintest stars.

- The burst reached its peak brightness sometime between 1,000 and 2,600 seconds, then faded gradually until it fell below the ability of TESS to detect it some 7000 seconds after it first went off.

How SMU and a team of exoplanet specialists confirmed the burst

- This gamma-ray burst was first detected by a NASA’s satellite called Swift-BAT, which was built to find these bursts. But because GRB 191016A occurred too close to the moon, the Swift-BAT couldn’t do the necessary follow-up it normally would have to learn more about it until hours later.

- NASA’s TESS happened to be looking at that same part of the sky. That was sheer luck, as TESS turns its attention to a new strip of the sky every month.

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Figure 19: TESS full-frame image in the cadence just before the BAT trigger (left) and at the peak flux of the burst (center). The emergence of the afterglow is apparent in the center of the image, indicated by the white arrow. The right panel shows the same region of the sky, with a slightly different orientation, in the Digitized Sky Survey (DSS); a small inset of TESS image is provided in the bottom left corner to demonstrate the change in orientation (image credit: SMU and collaborators)

- While exoplanet researchers at a ground-base for TESS could tell right away that a gamma-ray burst had happened, it would be months before they got any data from the TESS satellite on it. But since their focus was on new planets, these researchers asked if any other scientists at a TESS conference in Sydney, Australia was interested in doing more digging on the blast.

- Smith was one of the few high-energy astrophysics specialists there at that time and quickly volunteered.

- “The TESS satellite has a lot of potential for high-energy applications, and this was too good an example to pass up,” she said. High-energy astrophysics studies the behavior of matter and energy in extreme environments, including the regions around black holes, powerful relativistic jets, and explosions like gamma ray bursts.

- TESS is an optical telescope that collects light curves on everything in its field of view, every half hour. Light curves are a graph of light intensity of a celestial object or region as a function of time. Smith analyzed three of these light curves to be able to determine how bright the burst was.

- She also used data from ground-based observatories and the Swift gamma ray satellite to determine the burst's distance and other qualities about it.

- "Because the burst reached its peak brightness later and had a peak brightness that was higher than most bursts, it allowed the TESS telescope to make multiple observations before the burst faded below the telescope's detection limit," Smith said. “We’ve provided the only space-based optical follow-up on this exceptional burst.”

• February 12, 2021: Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a trio of hot worlds larger than Earth orbiting a much younger version of our Sun called TOI 451. The system resides in the recently discovered Pisces-Eridanus stream, a collection of stars less than 3% the age of our solar system that stretches across one-third of the sky. 26)

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Figure 20: This illustration sketches out the main features of TOI 451, a triple-planet system located 400 light-years away in the constellation Eridanus (image credit: NASA’s Goddard Space Flight Center)

- The planets were discovered in TESS images taken between October and December 2018. Follow-up studies of TOI 451 and its planets included observations made in 2019 and 2020 using NASA’s Spitzer Space Telescope, which has since been retired, as well as many ground-based facilities. Archival infrared data from NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellite – collected between 2009 and 2011 under its previous moniker, WISE – suggests the system retains a cool disk of dust and rocky debris. Other observations show that TOI 451 likely has two distant stellar companions circling each other far beyond the planets.

- “This system checks a lot of boxes for astronomers,” said Elisabeth Newton, an assistant professor of physics and astronomy at Dartmouth College in Hanover, New Hampshire, who led the research. “It’s only 120 million years old and just 400 light-years away, allowing detailed observations of this young planetary system. And because there are three planets between two and four times Earth’s size, they make especially promising targets for testing theories about how planetary atmospheres evolve.”

- A paper reporting the findings was published on Jan. 14 in The Astronomical Journal and is available online.

- Stellar streams form when the gravity of our Milky Way galaxy tears apart star clusters or dwarf galaxies. The individual stars move out along the cluster’s original orbit, forming an elongated group that gradually disperses.

- In 2019, a team led by Stefan Meingast at the University of Vienna used data from the European Space Agency’s Gaia mission to discover the Pisces-Eridanus stream, named for the constellations containing the greatest concentrations of stars. Stretching across 14 constellations, the stream is about 1,300 light-years long. However, the age initially determined for the stream was much older than we now think.

- Later in 2019, researchers led by Jason Curtis at Columbia University in New York City analyzed TESS data for dozens of stream members. Younger stars spin faster than their older counterparts do, and they also tend to have prominent starspots – darker, cooler regions like sunspots. As these spots rotate in and out of our view, they can produce slight variations in a star’s brightness that TESS can measure.

- The TESS measurements revealed overwhelming evidence of starspots and rapid rotation among the stream’s stars. Based on this result, Curtis and his colleagues found that the stream was only 120 million years old – similar to the famous Pleiades cluster and eight times younger than previous estimates. The mass, youth, and proximity of the Pisces-Eridanus stream make it an exciting fundamental laboratory for studying star and planet formation and evolution.

- “Thanks to TESS’s nearly all-sky coverage, measurements that could support a search for planets orbiting members of this stream were already available to us when the stream was identified,” said Jessie Christiansen, a co-author of the paper and the deputy science lead at the NASA Exoplanet Archive, a facility for researching worlds beyond our solar system managed by Caltech in Pasadena, California. “TESS data will continue to allow us to push the limits of what we know about exoplanets and their systems for years to come.”

- The young star TOI 451, better known to astronomers as CD-38 1467, lies about 400 light-years away in the constellation Eridanus. It has 95% of our Sun’s mass, but it is 12% smaller, slightly cooler, and emits 35% less energy. TOI 451 rotates every 5.1 days, which is more than five times faster than the Sun.

- TESS spots new worlds by looking for transits, the slight, regular dimmings that occur when a planet passes in front of its star from our perspective. Transits from all three planets are evident in the TESS data. Newton’s team obtained measurements from Spitzer that supported the TESS findings and helped to rule out possible alternative explanations. Additional follow-up observations came from Las Cumbres Observatory – a global telescope network headquartered in Goleta, California – and the Perth Exoplanet Survey Telescope in Australia.

- Even TOI 451’s most distant planet orbits three times closer than Mercury ever approaches to the Sun, so all of these worlds are quite hot and inhospitable to life as we know it. Temperature estimates range from about 2,200 degrees Fahrenheit (1,200 degrees Celsius) for the innermost planet to about 840 F (450 C) for the outermost one.

- TOI 451 b orbits every 1.9 days, is about 1.9 times Earth’s size, and its estimated mass ranges from two to 12 times Earth’s. The next planet out, TOI 451 c, completes an orbit every 9.2 days, is about three times larger than Earth, and holds between three and 16 times Earth’s mass. The farthest and largest world, TOI 451 d, circles the star every 16 days, is four times the size of our planet, and weighs between four and 19 Earth masses.

- Astronomers expect planets as big as these to retain much of their atmospheres despite the intense heat from their nearby star. Different theories of how atmospheres evolve by the time a planetary system reaches TOI 451’s age predict a wide range of properties. Observing starlight passing through the atmospheres of these planets provides an opportunity to study this phase of development and could aid in constraining current models.

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Figure 21: The Pisces-Eridanus stream spans 1,300 light-years, sprawling across 14 constellations and one-third of the sky. Yellow dots show the locations of known or suspected members, with TOI 451 circled. TESS observations show that the stream is about 120 million years old, comparable to the famous Pleiades cluster in Taurus (upper left), image credit: NASA’s Goddard Space Flight Center

- “By measuring starlight penetrating a planet’s atmosphere at different wavelengths, we can infer its chemical composition and the presence of clouds or high-altitude hazes,” said Elisa Quintana, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “TOI 451’s planets offer excellent targets for such studies with Hubble and the upcoming James Webb Space Telescope.”

- Observations from WISE show that the system is unusually bright in infrared light, which is invisible to human eyes, at wavelengths of 12 and 24 micrometers. This suggests the presence of a debris disk, where rocky asteroid-like bodies collide and grind themselves to dust. While Newton and her team cannot determine the extent of the disk, they envision it as a diffuse ring of rock and dust centered about as far from the star as Jupiter is from our Sun.

- The researchers also investigated a faint neighboring star that appears about two pixels away from TOI 451 in TESS images. Based on Gaia data, Newton’s team determined this star to be a gravitationally bound companion located so far from TOI 451 that its light takes 27 days to get there. In fact, the researchers think the companion is likely a binary system of two M-type dwarf stars, each with about 45% of the Sun’s mass and emitting only 2% of its energy.

- 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 Center for Astrophysics | Harvard & Smithsonian 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 Jet Propulsion Laboratory in Southern California manages NEOWISE for NASA's Science Mission Directorate in Washington. Ball Aerospace & Technologies Corp. of Boulder, Colorado, built the spacecraft. Science data processing takes place at IPAC at Caltech in Pasadena. Caltech manages JPL for NASA.

• January 27, 2021: The discovery: TYC 7037-89-1 is the first six-star system ever found where all of the stars participate in eclipses, a discovery made by NASA’s Transiting Exoplanet Survey Satellite (TESS). The system is located about 1,900 light-years away in the constellation Eridanus. 27)

- Key facts: The system, also called TIC 168789840, is the first known sextuple composed of three sets of eclipsing binaries, stellar pairs whose orbits tip into our line of sight so we observe the stars alternatively passing in front of each other. Each eclipse causes a dip in the system’s overall brightness. Astronomers designate the binaries by the letters A, B, and C. The stars in the A and C systems orbit each other roughly every day and a half, and the two binaries orbit each other about every four years. The B binary’s members circle each other about every eight days, but the pair is much farther away, orbiting around the inner systems roughly every 2,000 years. The primary stars in all three binaries are all slightly bigger and more massive than the Sun and about as hot. The secondaries are all around half the Sun’s size and a third as hot.

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Figure 22: This schematic shows the configuration of the sextuple star system TYC 7037-89-1. The inner quadruple is composed of two binaries, A and C, which orbit each other every four years or so. An outer binary, B, orbits the quadruple roughly every 2,000 years. All three pairs are eclipsing binaries. The orbits shown are not to scale (image credit: NASA's Goddard Space Flight Center)

- Details: Scientists used the NASA Center for Climate Simulation’s Discover supercomputer at NASA/GSFC to chart how the brightness of around 80 million stars observed by TESS changed over time. They then analyzed the data using autonomous software trained to recognize the tell-tale brightness dips of eclipsing binaries. Among the 450,000 candidates, researchers identified at least 100 with potentially three or more stars, including the new sextuple system.

- Fun facts: Astrophysicists are very interested in eclipsing binaries because their structure aids detailed measurements of the stars’ sizes, masses, temperatures, and separation as well as the distance to the system. They can use this information to build better models of star formation and evolution. For example, in the case of TYC 7037-89-1, scientists want to learn more about how the primary and secondary stars across the three binaries developed such similar properties and how the three systems became gravitationally bound.

- The discoverers: An international team, led by data scientist Brian P. Powell and astrophysicist Veselin Kostov at Goddard, made the discovery using TESS data. The researchers incorporated archival measurements and also obtained follow-up observations with ground-based facilities. The core team includes Saul Rappaport at MIT, Tamás Borkovits at the University of Szeged in Hungary, Petr Zasche at Charles University in the Czech Republic, and Andrei Tokovinin at NSF’s NOIRLab. 28)

• January 12, 2021: During a typical year, over a million people visit Yellowstone National Park, where the Old Faithful geyser regularly blasts a jet of boiling water high in the air. Now, an international team of astronomers has discovered a cosmic equivalent, a distant galaxy that erupts roughly every 114 days. 29)

- Using data from facilities including NASA’s Neil Gehrels Swift Observatory and Transiting Exoplanet Survey Satellite (TESS), the scientists have studied 20 repeated outbursts of an event called ASASSN-14ko. These various telescopes and instruments are sensitive to different wavelengths of light. By using them collaboratively, scientists obtained more detailed pictures of the outbursts.

- “These are the most predictable and frequent recurring multiwavelength flares we’ve seen from a galaxy’s core, and they give us a unique opportunity to study this extragalactic Old Faithful in detail,” said Anna Payne, a NASA Graduate Fellow at the University of Hawai’i at Mānoa. “We think a supermassive black hole at the galaxy’s center creates the bursts as it partially consumes an orbiting giant star.”

- Payne presented the findings on Tuesday, Jan. 12, at the virtual 237th meeting of the American Astronomical Society. A paper on the source and these observations, led by Payne, is undergoing scientific review.

Figure 23: Watch as a monster black hole partially consumes an orbiting giant star. In this illustration, the gas pulled from the star collides with the black hole’s debris disk and causes a flare. Astronomers have named this repeating event ASASSN-14ko. The flares are the most predictable and frequent yet seen from an active galaxy (video credit: NASA’s Goddard Space Flight Center)

- Astronomers classify galaxies with unusually bright and variable centers as active galaxies. These objects can produce much more energy than the combined contribution of all their stars, including higher-than-expected levels of visible, ultraviolet, and X-ray light. Astrophysicists think the extra emission comes from near the galaxy’s central supermassive black hole, where a swirling disk of gas and dust accumulates and heats up because of gravitational and frictional forces. The black hole slowly consumes the material, which creates random fluctuations in the disk’s emitted light.

- But astronomers are interested in finding active galaxies with flares that happen at regular intervals, which might help them identify and study new phenomena and events.

- “ASASSN-14ko is currently our best example of periodic variability in an active galaxy, despite decades of other claims, because the timing of its flares is very consistent over the six years of data Anna and her team analyzed,” said Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who studies black holes but was not involved in the research. “This result is a real tour de force of multiwavelength observational astronomy.”

- ASASSN-14ko was first detected on Nov. 14, 2014, by the All-Sky Automated Survey for Supernovae (ASAS-SN), a global network of 20 robotic telescopes headquartered at Ohio State University (OSU) in Columbus. It occurred in ESO 253-3, an active galaxy over 570 million light-years away in the southern constellation Pictor. At the time, astronomers thought the outburst was most likely a supernova, a one-time event that destroys a star.

- Six years later, Payne was examining ASAS-SN data on known active galaxies as part of her thesis work. Looking at the ESO 253-3 light curve, or the graph of its brightness over time, she immediately noticed a series of evenly spaced flares – a total of 17, all separated by about 114 days. Each flare reaches its peak brightness in about five days, then steadily dims.

- Payne and her colleagues predicted that the galaxy would flare again on May 17, 2020, so they coordinated joint observations with ground- and space-based facilities, including multiwavelength measurements with Swift. ASASSN-14ko erupted right on schedule. The team has since predicted and observed subsequent flares on Sept. 7 and Dec. 20.

- The researchers also used TESS data for a detailed look at a previous flare. TESS observes swaths of the sky called sectors for about a month at a time. During the mission’s first two years, the cameras collected a full sector image every 30 minutes. These snapshots allowed the team to create a precise timeline of a flare that began on Nov. 7, 2018, tracking its emergence, rise to peak brightness, and decline in great detail.

- “TESS provided a very thorough picture of that particular flare, but because of the way the mission images the sky, it can’t observe all of them,” said co-author Patrick Vallely, an ASAS-SN team member and National Science Foundation graduate research fellow at OSU. “ASAS-SN collects less detail on individual outbursts, but provides a longer baseline, which was crucial in this case. The two surveys complement one another.”

- Using measurements from ASAS-SN, TESS, Swift and other observatories, including NASA’s NuSTAR and the European Space Agency’s XMM-Newton, Payne and her team came up with three possible explanations for the repeating flares.

- One scenario involved interactions between the disks of two orbiting supermassive black holes at the galaxy’s center. Recent measurements, also under scientific review, suggest the galaxy does indeed host two such objects, but they don’t orbit closely enough to account for the frequency of the flares.

- The second scenario the team considered was a star passing on an inclined orbit through a black hole’s disk. In that case, scientists would expect to see asymmetrically shaped flares caused when the star disturbs the disk twice, on either side of the black hole. But the flares from this galaxy all have the same shape.

- The third scenario, and the one the team thinks most likely, is a partial tidal disruption event.

- A tidal disruption event occurs when an unlucky star strays too close to a black hole. Gravitational forces create intense tides that break the star apart into a stream of gas. The trailing part of the stream escapes the system, while the leading part swings back around the black hole. Astronomers see bright flares from these events when the shed gas strikes the black hole’s accretion disk.

- In this case, the astronomers suggest that one of the galaxy’s supermassive black holes, one with about 78 million times the Sun’s mass, partially disrupts an orbiting giant star. The star’s orbit isn’t circular, and each time it passes closest to the black hole, it bulges outward, shedding mass but not completely breaking apart. Every encounter strips away an amount of gas equal to about three times the mass of Jupiter.

- Astronomers don’t know how long the flares will persist. The star can’t lose mass forever, and while scientists can estimate the amount of mass it loses during each orbit, they don’t know how much it had before the disruptions began.

- Payne and her team plan to continue observing the event’s predicted outbursts, including upcoming dates in April and August 2021. They’ll also be able to examine another measurement from TESS, which captured the Dec. 20 flare with its updated 10-minute snapshot rate.

- “TESS was primarily designed to find worlds beyond our solar system,” said Padi Boyd, the TESS project scientist at Goddard. “But the mission is also teaching us more about stars in our own galaxy, including how they pulse and eclipse each other. In distant galaxies, we’ve seen stars end their lives in supernova explosions. TESS has even previously observed a complete tidal disruption event. We’re always looking forward to the next exciting and surprising discoveries the mission will make.”

- 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.

- Goddard 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 in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy.