TOI-1801 b: A temperate mini-Neptune around a young M0.5 dwarf

¹ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: mmd@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
⁴ Dipartimento di Fisica, Universita degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁵ Department of Astronomy, 501 Campbell Hall, University of California at Berkeley, Berkeley, CA 94720, USA
⁶ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁷ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117, Heidelberg, Germany
⁹ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008, Granada, Spain
¹¹ Centro de Astrobiología (CAB), (CSIC-INTA), ESAC Campus, Camino bajo del castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
¹² Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
¹³ NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA 91106, USA
¹⁴ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ Institut d’Estudis Espacials de Catalunya (IEEC), Calle Gran Capita 2-4, 08034 Barcelona, Spain
¹⁶ Public observatory ASTROLAB IRIS, Provinciaal Domein “De Palingbeek”, Verbrandemolenstraat 5, 8902 Zillebeke, Ieper, Belgium
¹⁷ Vereniging Voor Sterrenkunde (VVS), Oostmeers 122 C, 8000 Brugge, Belgium
¹⁸ Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
¹⁹ Department of Physics and Astronomy, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
²⁰ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²¹ Steward Observatory and Department of Astronomy, The University of Arizona, Tucson, AZ 85721, USA
²² Institut de Ciéncies de l’Espai (CSIC-IEEC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
²³ Departamento de Fśica de la Tierra y Astrofísica and IPARCOSUCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
²⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
²⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁶ Centro Astronómico Hispano en Andalucía (CAHA), Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Spain
²⁷ Department of Astronomy and Astrophysics, University of California, Santa Cruz, Santa Cruz, CA, USA
²⁸ Department of Physics & Astronomy, The University of California, Irvine, Irvine, CA 92697, USA
²⁹ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³⁰ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
³¹ Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA
³² Caltech IPAC - NASA Exoplanet Science Institute 1200 E. California Ave, Pasadena, CA 91125, USA
³³ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA
³⁴ Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA
³⁵ Division of Geological and Planetary Sciences, 1200 E California Blvd, Pasadena, CA 91125, USA
³⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³⁷ NASA Ames Research Center, Moffett Field, CA 94035, USA
³⁸ William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, 3400 N Charles St, Baltimore, MD 21218, USA
³⁹ Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA
⁴⁰ Villa ’39 Observatory, Landers, CA 92285, USA
⁴¹ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019, USA
⁴² Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
⁴³ Mississippi State University, 355 Lee Boulevard, Mississippi State, MS 39762, USA
⁴⁴ Department of Physics and Astronomy, University of Notre Dame, Notre Dame, IN 46556, USA
⁴⁵ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
⁴⁶ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
⁴⁷ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴⁸ Instituto de Astrofisica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁴⁹ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan

Received: 3 July 2023
Accepted: 4 October 2023

Abstract

We report the discovery, mass, and radius determination of TOI-1801 b, a temperate mini-Neptune around a young M dwarf. TOI-1801 b was observed in TESS sectors 22 and 49, and the alert that this was a TESS planet candidate with a period of 21.3 days went out in April 2020. However, ground-based follow-up observations, including seeing-limited photometry in and outside transit together with precise radial velocity (RV) measurements with CARMENES and HIRES revealed that the true period of the planet is 10.6 days. These observations also allowed us to retrieve a mass of 5.74 ± 1.46 M_⊕, which together with a radius of 2.08 ± 0.12 R_⊕, means that TOI-1801 b is most probably composed of water and rock, with an upper limit of 2% by mass of H₂ in its atmosphere. The stellar rotation period of 16 days is readily detectable in our RV time series and in the ground-based photometry. We derived a likely age of 600–800 Myr for the parent star TOI-1801, which means that TOI-1801 b is the least massive young mini-Neptune with precise mass and radius determinations. Our results suggest that if TOI-1801 b had a larger atmosphere in the past, it must have been removed by some evolutionary mechanism on timescales shorter than 1 Gyr.