The young mini-Neptune HD 207496b that is either a naked core or on the verge of becoming one

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: susana.barros@astro.up.pt
² Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁵ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
⁶ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA
⁷ Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
⁸ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA
⁹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
¹¹ Hazelwood Observatory, Australia
¹² Wild Boar Remote Observatory, San Casciano in val di Pesa, Firenze 50026, Italy
¹³ Tsinghua International School, Beijing 100084, PR China
¹⁴ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
¹⁵ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁶ U.S. Naval Observatory, 3450 Massachusetts Avenue NW, Washington, DC 20392, USA
¹⁷ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
¹⁸ Département d’astronomie de l’Université de Genève, Chemin Pegasi, 51, 1290 Sauverny, Switzerland
¹⁹ Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus, 28692, Villanueva de la Cañada (Madrid), Spain
²⁰ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
²¹ Institut Universitaire de France, IUF  
²² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²⁴ SETI Institute, Mountain View, CA 94043, USA
²⁵ Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁶ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁷ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland

Received: 20 December 2022
Accepted: 1 March 2023

Abstract

Aims. We report the discovery and characterisation of the transiting mini-Neptune HD 207496 b (TOI-1099) as part of a large programme that aims to characterise naked core planets.

Methods. We obtained HARPS spectroscopic observations, one ground-based transit, and high-resolution imaging which we combined with the TESS photometry to confirm and characterise the TESS candidate and its host star.

Results. The host star is an active early K dwarf with a mass of 0.80 ± 0.04 M_⊙, a radius of 0.769 ± 0.026 R_⊙, and a G magnitude of 8. We found that the host star is young, ~0.52 Gyr, allowing us to gain insight into planetary evolution. We derived a planetary mass of 6.1 ± 1.6 M_⊕, a planetary radius of 2.25 ± 0.12 R_⊕, and a planetary density of ρ_p = 3.27_−0.91^+0.97 g cm⁻³.

Conclusions. From internal structure modelling of the planet, we conclude that the planet has either a water-rich envelope, a gas-rich envelope, or a mixture of both. We have performed evaporation modelling of the planet. If we assume the planet has a gas-rich envelope, we find that the planet has lost a significant fraction of its envelope and its radius has shrunk. Furthermore, we estimate it will lose all its remaining gaseous envelope in ~0.52 Gyr. Otherwise, the planet could have already lost all its primordial gas and is now a bare ocean planet. Further observations of its possible atmosphere and/or mass-loss rate would allow us to distinguish between these two hypotheses. Such observations would determine if the planet remains above the radius gap or if it will shrink and be below the gap.