TOI-6508 b: A massive transiting brown dwarf orbiting a low-mass star

¹ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
² Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁴ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁵ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁶ Department of Astronomy & Astrophysics, UC San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
⁷ Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁹ Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
¹⁰ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Faculty of sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco
¹¹ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹² Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, B-4000 Liège, Belgium
¹³ NASA Exoplanet Science Institute, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
¹⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁶ Paris Region Fellow, Marie Sklodowska-Curie Action, France
¹⁷ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif-sur-Yvette, France
¹⁸ Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
¹⁹ Department of Astronomy, Tsinghua University, Beijing 100084, PR China
²⁰ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, Ciudad de México 04510, Mexico
²¹ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
²² Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA
²³ NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
²⁵ Institute for Particle Physics and Astrophysics , ETH Zürich, Wolfgang-Pauli-Strasse 2, 8093 Zürich, Switzerland
²⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
²⁷ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁸ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
²⁹ Hazelwood Observatory, Australia
³⁰ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

^★ Corresponding author; khalid.barkaoui@uliege.be

Received: 18 December 2024
Accepted: 26 February 2025

Abstract

We report the discovery of a transiting brown dwarf orbiting a low-mass star, TOI-6508 b. Today, only ∼50 transiting brown dwarfs have been discovered. TOI-6508 b was first detected with data from the Transiting Exoplanet Survey Satellite (TESS) in Sectors 10, 37 and 63. Ground-based follow-up photometric data were collected with the SPECULOOS-South (Search for habitable Planets EClipsing ULtra-cOOl Stars) and LCOGT-1m telescopes, and RV measurements were obtained with the Near InfraRed Planet Searcher (NIRPS) spectrograph. We find that TOI-6508 b has a mass of M_p = 72.5_−5.1^+7.6 M_Jup and a radius of R_p = 1.03 ± 0.03 R_Jup. Our modeling shows that the data are consistent with an eccentric orbit of 19 day and an eccentricity of e = 0.28_−0.08^+0.09. TOI-6508 b has a mass ratio of M_BD/M_★ = 0.40, makes it the second highest mass ratio brown dwarf that transits a low-mass star. The host has a mass of M_★ = 0.174 ± 0.004 M_⊙, a radius of R_★ = 0.205 ± 0.006 R_⊙, an effective temperature of T_eff = 2930 ± 70 K, and a metallicity of [Fe/H] = −0.22 ± 0.08. This makes TOI-6508 b an interesting discovery that has come to light in a region still sparsely populated.