K2-138b

K2-138b is a potentially rocky Super-Earth exoplanet orbiting every 2 days around a K1V star. The planet, along with the four others in the system, was found by citizen scientists of the Exoplanet Explorers project on Zooniverse. It was the final planet found in the system and was officially announced on January 8, 2018.

K2-138b

Discovery
Discovered by	Jessie L. Christiansen et al.[1]
Discovery site	Kepler
Discovery date	January 11, 2018
Detection method	Transit method
Orbital characteristics
Epoch BJD 2457773.3170[1]
Semi-major axis	0.03380 AU (5,056,000 km)[1]
Eccentricity	<0.403[1]
Orbital period (sidereal)	2.35322[1] d
Inclination	86.9[1]
Star	K2-138
Physical characteristics
Mean radius	1.57[1] R🜨
Mass	3.1+1.1 −1.1[2] ME
Mean density	4.9+2.0 −1.8[2] g/cm−3

K2-138b is the smallest planet of K2-138 with a radius of 1.57 R_🜨, meaning it could be rocky. It orbits its host star every 2.35 days at a distance of 0.0338 AU. At this proximity, the planet is likely very hot and receives 486 times the stellar flux as Earth.^[1][3] The planet has a Mass of 3.1+1.1
−1.1 M_E and a bulk density of 4.9+2.0
−1.8 g/cm⁻³, which is an earth-like density. The planet has likely a rocky core and a substantial atmospheric layer, composed of volatiles.^[2]

The K2-138 system is unique for being the first exoplanet system discovered entirely by citizen scientists.^[4][5][6]

The K2-138 system, including K2-138b will be studied with CHEOPS to further constrain the mass of the planets with transit-timing variation (TTV).^[7]

See also

• List of exoplanets discovered in 2017

References

1. Christiansen, Jessie L.; Crossfield, Ian J. M.; Barentsen, Geert; Lintott, Chris J.; Barclay, Thomas; Simmons, Brooke D.; Petigura, Erik; Schlieder, Joshua E.; Dressing, Courtney D.; Vanderburg, Andrew; Allen, Campbel; McMaster, Adam; Miller, Grant; Veldthuis, Martin; Allen, Sarah; Wolfenbarger, Zach; Cox, Brian; Zemiro, Julia; Howard, Andrew W.; Livingston, John; Sinukoff, Evan; Catron, Timothy; Grey, Andrew; Kusch, Joshua J. E.; Terentev, Ivan; Vales, Martin; Kristiansen, Martti H. (11 January 2018). "The K2-138 System: A Near-Resonant Chain of Five Sub-Neptune Planets Discovered by Citizen Scientists". The Astronomical Journal. 155 (2): 57. arXiv:1801.03874. Bibcode:2018AJ....155...57C. doi:10.3847/1538-3881/aa9be0. S2CID 52971376.
2. Lopez, T. A.; Barros, S. C. C.; Santerne, A.; Deleuil, M.; Adibekyan, V.; Almenara, J.-M.; Armstrong, D. J.; Brugger, B.; Barrado, D.; Bayliss, D.; Boisse, I.; Bonomo, A. S.; Bouchy, F.; Brown, D. J. A.; Carli, E.; Demangeon, O.; Dumusque, X.; Díaz, R. F.; Faria, J. P.; Figueira, P.; Foxell, E.; Giles, H.; Hébrard, G.; Hojjatpanah, S.; Kirk, J.; Lillo-Box, J.; Lovis, C.; Mousis, O.; da Nóbrega, H. J.; Nielsen, L. D.; Neal, J. J.; Osborn, H. P.; Pepe, F.; Pollacco, D.; Santos, N. C.; Sousa, S. G.; Udry, S.; Vigan, A.; Wheatley, P. J. (2019-11-01). "Exoplanet characterisation in the longest known resonant chain: the K2-138 system seen by HARPS". Astronomy & Astrophysics. 631: A90. arXiv:1909.13527. Bibcode:2019A&A...631A..90L. doi:10.1051/0004-6361/201936267. ISSN 0004-6361.
3. "K2-138 b". exoplanetarchive.ipac.caltech.edu.
4. "Five-Exoplanet System Discovered Thanks to Citizen Scientists". Space.com. 14 January 2018.
5. "Citizen Scientists Discover Five-Planet System - Caltech". Archived from the original on 2018-09-09. Retrieved 2018-01-19.
6. Daley, Jason. "Citizen Scientists Discover an Unusual Five-Planet Solar System".
7. "AO-1 Programmes - CHEOPS Guest Observers Programme - Cosmos". www.cosmos.esa.int. Retrieved 2019-11-11.