Summary：
A research team led by researchers from the University of Tokyo and the Astrobiology Center of the National Institutes of Natural Sciences has discovered a new exoplanet “TOI-2285b” located near the solar system (138 light-years away) through a collaboration between the exoplanet exploration satellite TESS and ground-based telescopes. This planet is approximately 1.7 times the size (radius) of Earth and receives about 1.5 times the amount of irradiance that Earth receives from the Sun from its host star, which is weaker than most of previously discovered exoplanets. The planet is believed to have a slightly warmer environment than Earth, and if it has a layer of H2O in its interior and a hydrogen-dominated atmosphere, liquid water could exist on its surface. Detailed follow-up observations are feasible as the host star is bright, and future investigations into the planet’s mass and atmospheric composition are expected to provide detailed information about its internal composition.
Research Background：
More than 4,000 exoplanets have been discovered by the Kepler Space Telescope of the National Aeronautics and Space Administration (NASA), which was active from 2009-2018, using the transit method (Note 2). These include many warm and small exoplanets that are expected to harbor life (Figure 1). However, most of the planetary systems discovered by the Kepler space telescope are located more than 500 light years away from the solar system, and it has been difficult to obtain detailed information such as the mass and atmospheric composition of the planets because their main stars are faint. The TESS space telescope, the successor to the Kepler space telescope, is currently searching for exoplanets around bright stars in the entire sky, and it is expected that subsequent follow-up observations of planets around bright stars discovered by the TESS search will provide detailed information on the mass and atmospheric composition of the planets. The TESS search for exoplanets around bright stars is expected to yield detailed information on the planet’s mass and atmospheric composition through follow-up observations.
On the other hand, due to limitations such as resolution and observation period, TESS observations alone can only discover “candidate” planetary objects. Therefore, in order to discover true planets, it is necessary to verify the authenticity of the discovered planetary candidates through detailed observations using ground-based telescopes. Therefore, a research team led by researchers from the University of Tokyo and the National Astrobiology Center of the National Institutes of Natural Sciences (NINS) is now using the MuSCAT series (Note 3) of multicolor imaging instruments mounted on three 2-meter class telescopes in Japan and abroad, and the infrared The IRD (*4) on the 8.2-meter Subaru Telescope in Hawaii is being used to verify planetary candidate objects discovered in the TESS search.
研究の成果：
The research team discovered TOI-2285b, a planet orbiting a star relatively close to our solar system (138 light-years away), from among the candidate planets they observed. It orbits around a low-temperature (3200 degrees Celsius) star with a period of about 27 days.
It is very important to observe the transit at multiple wavelengths in order to verify whether the candidate planet discovered by TESS is a real planet or not. However, since the transit of TOI-2285b occurs only once every 27 days, the opportunity to observe it from the ground under favorable conditions (nighttime and clear skies) was very limited. The research team developed three MuSCAT series instruments that can simultaneously observe the transit at multiple wavelengths and deployed them on three telescopes in Japan and abroad, which enabled them to confirm that TOI-2285b is a planet ahead of the rest of the world. Furthermore, by using the IRD, one of the world’s most accurate infrared Doppler instruments for measuring planetary masses, we succeeded in obtaining an upper limit for the mass of the planet (19 times the mass of the Earth).
The distance between TOI-2285b and the main star is only about 1/7 of the distance between the Earth and the Sun, but due to the low temperature of the main star, the amount of solar radiation the planet receives from the main star is estimated to be about 1.5 times that received by the Earth from the Sun. This insolation is moderate compared to many other exoplanets discovered so far, but it is still strong enough to quickly dry up the water on the planet’s surface if the planet were a rocky planet with a thin atmosphere like the Earth. On the other hand, if a layer of H2O exists outside the planet’s central core and a hydrogen-based atmosphere covers the outer layer (Note 5), part of the H2O layer may be stable as a liquid. The research team simulated the temperature and pressure inside TOI-2285b assuming such an internal composition, and found that there is indeed a possibility of liquid water (ocean) in the surface layer of the planet (top image).
Research Findings:
In order to determine whether liquid water actually exists in the surface layer of TOI-2285b in the future, it is important to first accurately measure the mass of the planet and then constrain the internal composition of the planet in combination with the already known information on the radius and insolation of the planet. To measure the mass of a planet, the main star must be bright enough, but since TOI-2285b is orbiting a star in the solar system’s neighborhood and appears bright in the infrared, it is possible to actually measure the mass using an infrared Doppler instrument attached to a large telescope such as IRD. Although this study has only obtained an upper limit for the mass of the planet, further observations are expected to enable the precise measurement of the planet’s mass and a closer look at the planet’s internal composition. In addition, next-generation telescopes such as the James Webb Space Telescope (JWST), which is scheduled for launch in December 2021, are expected to investigate the composition of the planet’s atmosphere to determine whether water and other molecules exist in the atmosphere.
The discovery of TOI-2285b is an important step toward the future “search for traces of life” on exoplanets. In the future, it is expected that next-generation large space telescopes and giant ground-based telescopes will be able to search for molecules such as water and oxygen in the atmospheres of warm exoplanets, which could be traces of life. On the other hand, in order to obtain reliable evidence of traces of life, it is not enough to observe only one or two planets; it is considered important to observe as many planets as possible. However, the number of promising planets (small, warm planets in the vicinity of the solar system) for observation is still very limited at this time (Figure 1). Since TESS is scheduled to continue its search until at least 2022, it is expected that the number of planets that are equal to or more promising than TOI-2285b can be further increased in the future by collaborating with ground-based telescopes as in this case. TOI-2285b or even more promising planets in the future.
The results of this research were published in the online edition of the journal “Publications of the Astronomical Society of Japan” on December 6, 2021. This research was conducted as part of the Grant-in-Aid for Scientific Research on Innovative Areas “Elucidation of the Formation and Evolution of Planetary Atmospheres and their Diversity” (PI: Dr. Taiyo Ikoma, Project Leader: JP18H05439) and the Japan Science and Technology Agency (JST) Strategic Basic Research Promotion Program “PRESTO” in the research area “Development and Application of Intelligent Measurement and Analysis Methodology by Integrating Measurement Technology and Advanced Information Processing.
JP17H04574), and the project “Discovery Confirmation and Characterization of Candidate Life-supporting Exoplanets Discovered by TESS” (PI: Noriyasu Narita, Project No.: AB031010) of the Center for Astrobiology, National Institutes of Natural Sciences, Japan.
Publication Journal：
Journal: Publications of the Astronomical Society of Japan (Online version: December 6)
Paper Title: TOI-2285b: A 1.7 Earth-radius Planet Near the Habitable Zone around a Nearby M Dwarf
Authors: Fukui, A.*, Kimura, T., Hirano, T., Narita, N., et al.
DOI: 10.1093/pasj/psab106
Terminology：
(Note 1) Officially known as the Transiting Exoplanet Survey Satellite, it was launched in 2018 to search for planets orbiting bright stars in the entire sky using the transit method (Note 2). The search is currently scheduled to continue until 2022.
(Note 2) This method captures the periodic dimming of the main star observed when a planet passes in front of the main star (transit).This method can be used to determine the radius and orbital period of a planet.
(Note 3) Instruments capable of simultaneously observing transits in three or four wavelength bands of visible light (named MuSCAT, MuSCAT2, and MuSCAT3, respectively) are installed on the 188cm telescope in Okayama Prefecture, a 1.52m aperture telescope in Tenerife, Spain, and a 2m aperture telescope in Maui, USA. MuSCAT2, and MuSCAT3, respectively). In this study, the transit signals observed by TESS were confirmed using MuSCAT2 and MuSCAT3.
(Note 4) An infrared spectrometer that can measure planetary masses with high precision using the Doppler method. By obtaining the upper limit of the mass, we confirmed that the transiting object is not a star but a planet (with a mass less than 13 times that of Jupiter).
(Note 5) The existence of an H2O layer in the interior of a planet is predicted from the theory of planet formation. On the other hand, it is highly likely that a hydrogen-based atmosphere existed at least in the early stages of planet formation, but it may have been stripped away by high-energy electromagnetic waves (X-rays and ultraviolet rays) emitted from the host star later on.
Related Links：
・The University of Tokyo Release
・Japan Science and Technology Agency release
