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

Abstract

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 $\pm$ 0.013 M${}_{\odot }$ and a radius of 0.516 $\pm$ 0.016 R${}_{\odot }$. We modelled the transit light curves and radial velocity measurements and obtained a planetary mass of 3.34 $\pm$ 0.72 M${}_{\oplus }$, a radius of 2.13 $\pm$ 0.11 R${}_{\oplus }$, 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 favourable 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${}_{-3.2}^{+8.9}$ M${}_{\oplus }$, whose transits are not detected.

25 pages, 26 figures, accepted for publication in A&A

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