GJ 3090 b: one of the most favourable mini-Neptune for atmospheric characterisation

Almenara, J. M.; Bonfils, X.; Otegi, J. F.; Attia, O.; Turbet, M.; Astudillo-Defru, N.; Collins, K. A.; Polanski, A. S.; Bourrier, V.; Hellier, C.; Ziegler, C.; Bouchy, F.; Briceno, C.; Charbonneau, D.; Cointepas, M.; Collins, K. I.; Crossfield, I.; Delfosse, X.; Diaz, R. F.; Dorn, C.; Doty, J. P.; Forveille, T.; Gaisné, G.; Gan, T.; Helled, R.; Hesse, K.; Jenkins, J. M.; Jensen, E. L. N.; Latham, D. W.; Law, N.; Mann, A. W.; Mao, S.; McLean, B.; Murgas, F.; Myers, G.; Seager, S.; Shporer, A.; Tan, T. G.; Twicken, J. D.; Winn, J.
Bibliographical reference

Astronomy and Astrophysics

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We report the detection of GJ 3090 b (TOI-177.01), a mini-Neptune on a 2.9-day orbit transiting a bright (K = 7.3 mag) M2 dwarf located at 22 pc. The planet was identified by the Transiting Exoplanet Survey Satellite and was confirmed with the High Accuracy Radial velocity Planet Searcher radial velocities. Seeing-limited photometry and speckle imaging rule out nearby eclipsing binaries. Additional transits were observed with the LCOGT, Spitzer, and ExTrA telescopes. We characterise the star to have a mass of 0.519 ± 0.013 M⊙ and a radius of 0.516 ± 0.016 R⊙. We modelled the transit light curves and radial velocity measurements and obtained a planetary mass of 3.34 ± 0.72 ME, a radius of 2.13 ± 0.11 RE, and a mean density of 1.89−0.45+0.52 g cm−3. The low density of the planet implies the presence of volatiles, and its radius and insolation place it immediately above the radius valley at the lower end of the mini-Neptune cluster. A coupled atmospheric and dynamical evolution analysis of the planet is inconsistent with a pure H-He atmosphere and favours a heavy mean molecular weight atmosphere. The transmission spectroscopy metric of 221−46+66 means that GJ 3090 b is the second or third most favorable mini-Neptune after GJ 1214 b whose atmosphere may be characterised. At almost half the mass of GJ 1214 b, GJ 3090 b is an excellent probe of the edge of the transition between super-Earths and mini-Neptunes. We identify an additional signal in the radial velocity data that we attribute to a planet candidate with an orbital period of 13 days and a mass of 17.1−3.2+8.9 ME, whose transits are not detected.