TOI-2180 b

TOI-2180 b is a giant exoplanet orbiting the G-type star TOI-2180, also known as HD 238894.[1] It was discovered with the help of the Transiting Exoplanet Survey Satellite and is currently the exoplanet with the longest orbital period TESS was able to uncover (as of September 2022).[3] TOI-2180 b orbits its host star every 260.16 days.[2]

TOI-2180 b
illustration of TOI-2180 b
Discovery
Discovered byThe TESS-Keck Survey
Discovery dateFebruary 2022
Transit Method
Designations
HD 238894 b
Orbital characteristics
0.828±0.012 au[1]
Eccentricity0.3683 ± 0.0073[1]
260.15764 ± 0.00045 days[2]
Inclination89.955+0.032
−0.044[1]
StarHD 238894
Physical characteristics
Mean radius
1.010+0.022
−0.019 RJ
Mass2.755+0.087
−0.081 MJ[1]
3.827+0.012
−0.015 dex[1]
Temperature348.0+3.3
−3.6 K[1]

    Discovery

    TOI-2180 b was first identified as an exoplanet candidate due to a single transit with TESS at 12./13. December 2019 by a group of citizen scientists called the Visual Survey Group, which includes Thomas Lee Jacobs, a former U.S. naval officer.[4] The group was using the light curve processing tool lcTools. In May 2020 the Planet Hunters: TESS collaboration announced this object as a Community TESS Object of Interest (CTOI) and it was soon promoted into a regular TOI.[1]

    The TESS-Keck Survey collaboration performed radial velocity follow-up observations for nearly 2 years with the Automated Planet Finder and Keck I. The follow-up observations uncovered that the single transit was caused by a long-period planet.[1]

    Orbital properties

    TOI-2180 b has a long orbital period of 260.16 days,[2] which also leads to a long transit duration of 24 hours.[1] The distance to the host star is 82.8% the sun-earth distance.[1] The planet does not orbit inside the habitable zone, despite this close resemblance in semi-major axis.[5] TOI-2180 b has a high eccentricity of the orbit at 0.37. The radial velocity monitoring also showed acceleration due to an outer planet or low-mass star in the system.[1]

    The second transit was not detected from the ground and the third transit was not observed.[1] The fourth transit was observed at 31. January/01. February 2022, refining the orbital period. The next transit will occur on 2022 October 18 at 21:28 UTC.[2]

    Physical properties

    The planet has the same size as Jupiter, but is 2.8 times heavier than Jupiter. TOI-2180 b stand out because of its cold estimated temperature of about 348 Kelvin (74.9 °C, 166.7 °F).[1] This is closer to Jupiters 165 K than most discovered giant exoplanets. TOI-2180 b belongs to a small sample of temperature Jupiters with a temperature <400 K that transit, such as Kepler-167 e, WD 1856+534 b, Kepler-1704 b, KOI-3680 b, Kepler-1514 b and Kepler-539 b. TOI-2180 b has by far the brightest host star with a visual magnitude of 9.16, which is about 3 magnitudes brighter than the next brightest system in this sample.[5]

    The planet is likely enriched in metals compared to its host star. The discovery team inferred that TOI-2180 b is enriched in metals by a factor of about 5 compared to its host star. This means it has about 100 M🜨 of heavy elements in its envelope and interior.[1]

    Future observations

    The exoplanet is a poor target for transmission spectroscopy because of its high surface gravity and the large radius of the host star. The large radius of the star causes a relative shallow transit depth of about 0.5%. The system is still an excellent target to find rings and exomoons around TOI-2180 b. It is also a good target to study the migration of exoplanets.[1]

    Host star

    The host star is a 9.16 magnitude[6] bright and slightly evolved star with a spectral type of G5. It has a mass of 1.1 M☉ and a radius of 1.6 R☉. The radius is increased due to the evolved nature of the star. The star is 116 parsec (379 light-years) distant from earth and has an age of about 8.1 billion years.[1]

    Habitable Zone

    Currently the habitable zone around TOI-2180 is between 1.5 and 2.2 astronomical units.[1][note 1] Because TOI-2180 is slightly evolved, it had a habitable zone closer to the star in the past. At an age of about 3 billion years the habitable zone was located between 1.1 and 1.6 astronomical units.[1][7][note 2]

    Notes
    1. Using a luminosity of 2.544 by Dalba et al. and using step 2 by Morris
    2. Using MIST evolutionary track with initial mass=1.115 Msun (best match from isochromes), initial metallicity [Fe/H]=0.269 (Delba et al for TOI-2180) and using synthetic photometry output UBV etc. Then the approach by Morris was used, assuming the star was an F-type back then.

    References
    1. Dalba, Paul A.; Kane, Stephen R.; Dragomir, Diana; Villanueva, Steven; Collins, Karen A.; Jacobs, Thomas Lee; LaCourse, Daryll M.; Gagliano, Robert; Kristiansen, Martti H.; Omohundro, Mark; Schwengeler, Hans M.; Terentev, Ivan A.; Vanderburg, Andrew; Fulton, Benjamin; Isaacson, Howard (2022-02-01). "The TESS-Keck Survey. VIII. Confirmation of a Transiting Giant Planet on an Eccentric 261 Day Orbit with the Automated Planet Finder Telescope". The Astronomical Journal. 163 (2): 61. arXiv:2201.04146. Bibcode:2022AJ....163...61D. doi:10.3847/1538-3881/ac415b. ISSN 0004-6256. S2CID 245877799.
    2. Dalba, Paul A.; Jacobs, Thomas Lee; Omohundro, Mark; Gagliano, Robert; Jursich, Jay; Kristiansen, Martti H.; LaCourse, Daryll M.; Schwengeler, Hans M.; Terentev, Ivan A. (2022-04-01). "The Refined Transit Ephemeris of TOI-2180 b". Research Notes of the American Astronomical Society. 6 (4): 76. Bibcode:2022RNAAS...6...76D. doi:10.3847/2515-5172/ac64fd. ISSN 2515-5172. S2CID 248150224.
    3. "Planetary Systems". exoplanetarchive.ipac.caltech.edu. Retrieved 2022-09-12.
    4. Landau, Elizabeth (2022-01-11). "Citizen Scientists Spot Jupiter-like Planet". NASA. Retrieved 2022-09-12.
    5. "Exoplanet-catalog". Exoplanet Exploration: Planets Beyond our Solar System. Retrieved 2022-09-12.
    6. Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000-03-01). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H. ISSN 0004-6361.
    7. "MIST". waps.cfa.harvard.edu. Retrieved 2022-09-24.
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