TOI-4438 b: a transiting mini-Neptune amenable to atmospheric characterization

¹ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
² Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
e-mail: elisa@tls-tautenburg.de
³ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁵ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁸ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁹ Centro de Astrobiología, (CSIC-INTA), ESAC Campus, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ Steward Observatory and Department of Astronomy, The University of Arizona, Tucson, AZ 85721, USA
¹¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
¹³ Max Planck Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹⁴ Departamento de Física de la Tierra y Astrofísica and IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁵ Department of Physics, Ariel University, Ariel 40700, Israel
¹⁶ Vereniging Voor Sterrenkunde, Oude Bleken 12, 2400 Mol, Belgium
¹⁷ AstroLAB IRIS, Provinciaal Domein “De Palingbeek”, Verbrande-molenstraat 5, 8902 Zillebeke, Ieper, Belgium
¹⁸ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹⁹ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²⁰ Institut d’Estudis Espacials de Catalunya (IEEC), Calle Gran Capita 2–4, 08034 Barcelona, Spain
²¹ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²² Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
²³ Centro Astronómico Hispano en Andalucía (CAHA), Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Spain
²⁴ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
²⁵ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
²⁶ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
²⁷ Institut de Ciéncies de l'Espai (CSIC-IEEC), Campus UAB, de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
²⁸ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
²⁹ Center for Astroparticles and High Energy Physics (CAPA), Universidad de Zaragoza, 50009 Zaragoza, Spain
³⁰ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus

Received: 29 December 2023
Accepted: 4 March 2024

Abstract

We report the confirmation and mass determination of a mini-Neptune transiting the M3.5 V star TOI-4438 (G 182-34) every 7.44 days. A transit signal was detected with NASA’s TESS space mission in the sectors 40, 52, and 53. In order to validate the planet TOI-4438 b and to determine the system properties, we combined TESS data with high-precision radial velocity measurements from the CARMENES spectrograph, spanning almost one year, and ground-based transit photometry. We found that TOI-4438 b has a radius of R_b = 2.52 ± 0.13 R_⊕ (5% precision), which together with a mass of M_b = 5.4 ± 1.1 M_⊕ (20% precision), results in a bulk density of ρ_b = 1.85_−0.44^+0.51 g cm⁻³ (~28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields aminimum water mass fraction of 46% (1σ). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO₂, and CH₄. The primary star has a J-band magnitude of 9.7, and the planet has a high transmission spectroscopy metric (TSM) of 136 ± 13. Taking into account the relatively warm equilibrium temperature of T_eq = 435 ± 15 K, and the low activity level of its host star, TOI-4438 b is one of the most promising mini-Neptunes around an M dwarf for transmission spectroscopy studies.