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

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: jose-manuel.almenara-villa@univ-grenoble-alpes.fr
² Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
³ Institute for Computational Science, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland
⁴ Departamento de Matemâtica y Fisica Aplicadas, Universidad Católica de la Santisima Concepción, Alonso de Rivera 2850, Concepción, Chile
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA
⁷ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
⁸ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA
⁹ SOAR Telescope/NSF’s NOIRLab, Casilla 603, La Serena, Chile
¹⁰ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
¹¹ International Center for Advanced Studies (ICAS) and ICIFI (CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, 1650 Buenos Aires, Argentina
¹² Noqsi Aerospace Ltd., 15 Blanchard Avenue, Billerica, MA 01821, USA
¹³ Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, PR China
¹⁴ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁵ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁶ Department of Physics & Astronomy, Swarthmore College, Swarthmore PA 19081, USA
¹⁷ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA
¹⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁹ Instituto de Astrofisica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
²⁰ Dept. Astrofisica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
²¹ AAVSO, 5 Inverness Way, Hillsborough, CA 94010, USA
²² Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²³ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
²⁴ Perth Exoplanet Survey Telescope, Perth, Western Australia
²⁵ Curtin Institute of Radio Astronomy, Curtin University, Bentley, Western Australia 6102, Australia
²⁶ SETI Institute, Mountain View, CA 94043, USA
²⁷ Department of Astrophysical Sciences, Princeton University, NJ 08544, USA

Received: 6 May 2022
Accepted: 14 July 2022

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 ± 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 M_E, a radius of 2.13 ± 0.11 R_E, 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 M_E, whose transits are not detected.