A transiting rocky super-Earth and a non-transiting sub-Neptune orbiting the M dwarf TOI-771

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom
⁴ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ INAF – Osservatorio Astronomico di Palermo, Piazza del Parla-mento 1, 90134 Palermo, Italy
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
⁸ Institut de Ciències de l’Espai (ICE,CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Barcelona, Spain
⁹ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzia̧dzka 5, 87-100 Toruń, Poland
¹¹ University of Texas at Austin, Department of Astronomy, 2515 Speedway C1400, Austin, TX 78712, USA
¹² Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹³ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹⁴ Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA,
¹⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁶ Department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁷ Magdalen College, University of Oxford, Oxford OX1 4AU, UK
¹⁸ Institute for Particle Physics and Astrophysics, ETH Zürich, Wolfgang-Pauli-Strasse 2, 8093 Zürich, Switzerland
¹⁹ Institute for Astronomy, University of Cambridge, Madingely Road, Cambridge, CB3 0HA, UK
²⁰ Mullard Space Science Laboratory, University College London, Surrey, UK
²¹ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK

^★ Corresponding author: glacedelli@iac.es

Received: 17 February 2025
Accepted: 24 April 2025

Abstract

Context. The origin and evolution of the sub-Neptune population is a highly debated topic in the exoplanet community. With the advent of JWST, atmospheric studies can now put unprecedented constraints on the internal composition of this population. In this context, the THIRSTEE project aims to investigate the population properties of sub-Neptunes with a comprehensive and demographic approach, providing a homogeneous sample of precisely characterised sub-Neptunes across stellar spectral types.

Aims. We present here the precise characterisation of the planetary system orbiting one of the THIRSTEE M-dwarf targets, TOI-771 (d = 25 pc, V = 14.9 mag), known to host one planet, TOI-771 b, which has been statistically validated using TESS observations.

Methods. We use TESS, SPECULOOS, TRAPPIST, and M-Earth photometry together with 31 high-precision ESPRESSO radial velocities to derive the orbital parameters and investigate the internal composition of TOI-771 b, as well as exploring the presence of additional companions in the system.

Results. We derived the precise mass and radius for TOI-771 b, a super-Earth with R_b = 1.36 ± 0.10 R_⊕ and M_b = 2.47_−0.31^+0.32M_⊕ orbiting every 2.3 days around its host star. Its composition is consistent with an Earth-like planet, and it adds up to the rocky population of subNeptunes lying below the density gap identified around M dwarfs. With a ∼ 13% precision in mass, a∼ 7% radius precision, and a warm equilibrium temperature of T_eq =543K, TOI-771 b is a particularly interesting target for atmospheric characterisation with JWST, and it is indeed one of the targets under consideration for the Rocky World DDT programme. Additionally, we discover the presence of a second, non-transiting planet in the system, TOI-771 c, with a period of 7.61 days and a minimum mass of M_p sin i = 2.87_−0.38^+0.41M_⊕. Even though the inclination is not directly constrained, the planet likely belongs to the temperate sub-Neptune population, with an equilibrium temperature of ∼ 365K.