TOI-5005 b: A super-Neptune in the savanna near the ridge

¹ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA
⁶ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁹ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, Spain
¹⁰ CFisUC, Departamento de Física, Universidade de Coimbra, 3004516 Coimbra, Portugal
¹¹ IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Université, 77 Av. Denfert-Rochereau, 75014 Paris, France
¹² INAF – Osservatorio Astrofisico di Torin, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹⁴ Astrobiology Research Unit, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
¹⁵ Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹⁶ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹⁷ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Faculty of sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco
¹⁸ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁹ Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
²⁰ SETI Institute, Mountain View, CA 94043 USA/NASA Ames Research Center, Moffett Field, CA 94035, USA
²¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²² Department of Physics and Astronomy, The University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA
²³ University of Maryland, Baltimore County, 1000 Hilltop Cir., Baltimore, MD 21250, USA
²⁴ NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
²⁵ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
²⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁷ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA
²⁸ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
²⁹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³⁰ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³¹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³² Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³³ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³⁴ Perth Exoplanet Survey Telescope, Perth, Western Australia
³⁵ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA

^★ Corresponding author; acastro@cab.inta-csic.es

Received: 25 July 2024
Accepted: 26 September 2024

Abstract

Context. The Neptunian desert and savanna have recently been found to be separated by a ridge, an overdensity of planets in the period range of ≃3–5 days. These features are thought to be shaped by dynamical and atmospheric processes. However, their roles are not yet well understood.

Aims. Our aim was to confirm and characterize the super-Neptune TESS candidate TOI-5005.01, which orbits a moderately bright (V = 11.8) solar-type star (G2 V) with an orbital period of 6.3 days. With these properties, TOI-5005.01 is located in the Neptunian savanna near the ridge.

Methods. We used Bayesian inference to analyse 38 HARPS radial velocity measurements, three sectors of TESS photometry, and two PEST and TRAPPIST-South transits. We tested a set of models involving eccentric and circular orbits, long-term drifts, and Gaussian processes to account for correlated stellar and instrumental noise. We computed the Bayesian evidence to find the model that best represents our dataset and infer the orbital and physical properties of the system.

Results. We confirm TOI-5005 b to be a transiting super-Neptune with a radius of R_p = 6.25 ± 0.24 R_⊕ (R_p = 0.558 ± 0.021 R_J) and a mass of M_p = 32.7 ± 5.9 M_⊕ (M_p = 0.103 ± 0.018 M_J), which corresponds to a mean density of ρ_p = 0.74 ± 0.16 g cm⁻³. Our internal structure modelling indicates that the core mass fraction (CMF = 0.74_−0.45^+0.05) and envelope metal mass fraction (Z_env = 0.08_−0.06^+0.41) of TOI-5005 b are degenerate, but the overall metal mass fraction is well constrained to a value slightly lower than that of Neptune and Uranus (Z_planet = 0.76_−0.11^+0.04). The Z_planet /Z_star ratio is consistent with the well-known mass-metallicity relation, which suggests that TOI-5005 b was formed via core accretion. We also estimated the present-day atmospheric mass-loss rate of TOI-5005 b, but found contrasting predictions depending on the choice of photoevaporation model (0.013 ± 0.008 M_⊕ Gyr⁻¹ vs. 0.17 ± 0.12 M_⊕ Gyr⁻¹). At a population level, we find statistical evidence (p-value = 0.0092_−0.0066^+0.0184) that planets in the savanna such as TOI-5005 b tend to show lower densities than planets in the ridge, with a dividing line around 1 g cm⁻³ , which supports the hypothesis of different evolutionary pathways populating the two regimes.

Conclusions. TOI-5005 b is located in a region of the period-radius space that is key to studying the transition between the Neptunian ridge and the savanna. It orbits the brightest star of all such planets known today, which makes it a target of interest for atmospheric and orbital architecture observations that will bring a clearer picture of its overall evolution.