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.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2022 vol. 665

0004-6361

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 Mâ ? ? and a radius of 0.516 ± 0.016 Râ ? ? . 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.