TOI-674b: An oasis in the desert of exo-Neptunes transiting a nearby M dwarf^★

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
e-mail: fmurgas@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
⁶ Department of Astronomy, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁷ Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
⁸ Observatoire de l’Université de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland
⁹ NASA Ames Research Center, Moffett Field, CA, 94035, USA
¹⁰ Caltech/IPAC, 1200 E. California Blvd. Pasadena, CA 91125, USA
¹¹ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹² U.S. Naval Observatory, Washington, DC 20392, USA
¹³ NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
¹⁶ Department of Physics & Astronomy, Swarthmore College, Swarthmore, PA 19081, USA
¹⁷ El Sauce Observatory, Coquimbo Province, Chile
¹⁸ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
¹⁹ Astrobiology Research Unit, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
²⁰ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
²¹ International Center for Advanced Studies (ICAS) and ICIFI(CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia (1650), Buenos Aires, Argentina
²² Université de Montréal, Département de Physique & Institut de Recherche sur les Exoplanètes, Montréal, QC H3C 3J7, Canada
²³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
²⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²⁵ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²⁶ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁷ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁸ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
²⁹ Department of Astrophysical Sciences, Princeton University, NJ 08544, USA
³⁰ SETI Institute, 189 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA
³¹ Space Telescope Science Institute, Baltimore, MD 21218, USA
³² Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA
³³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³⁴ Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA
³⁵ Department of Space, Earth and Environment, Astronomy and Plasma Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden

Received: 3 March 2021
Accepted: 1 June 2021

Abstract

Context. The NASA mission TESS is currently doing an all-sky survey from space to detect transiting planets around bright stars. As part of the validation process, the most promising planet candidates need to be confirmed and characterized using follow-up observations.

Aims. In this article, our aim is to confirm the planetary nature of the transiting planet candidate TOI-674b using spectroscopic and photometric observations.

Methods. We use TESS, Spitzer, ground-based light curves, and HARPS spectrograph radial velocity measurements to establish the physical properties of the transiting exoplanet candidate TOI-674b. We perform a joint fit of the light curves and radial velocity time series to measure the mass, radius, and orbital parameters of the candidate.

Results. We confirm and characterize TOI-674b, a low-density super-Neptune transiting a nearby M dwarf. The host star (TIC 158588995, V = 14.2 mag, J = 10.3 mag) is characterized by its M2V spectral type with M_⋆ = 0.420 ± 0.010 M_⊙, R_⋆ = 0.420 ± 0.013 R_⊙, and T_eff = 3514 ± 57 K; it is located at a distance d = 46.16 ± 0.03 pc. Combining the available transit light curves plus radial velocity measurements and jointly fitting a circular orbit model, we find an orbital period of 1.977143 ± 3 × 10⁻⁶ days, a planetary radius of 5.25 ± 0.17 R_⊕, and a mass of 23.6 ± 3.3 M_⊕ implying a mean density of ρ_p =0.91 ± 0.15 g cm⁻³. A non-circular orbit model fit delivers similar planetary mass and radius values within the uncertainties. Given the measured planetary radius and mass, TOI-674b is one of the largest and most massive super-Neptune class planets discovered around an M-type star to date. It is found in the Neptunian desert, and is a promising candidate for atmospheric characterization using the James Webb Space Telescope.