The CARMENES search for exoplanets around M dwarfs

Occurrence rates of Earth-like planets around very low-mass stars

¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
² Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Institut de Ciències de l’Espai (CSIC), c/ de Can Magrans s/n, Campus UAB, 08193 Bellaterra, Barcelona, Spain
⁶ Institut d’Estudis Espacials de Catalunya, 08860 Castelldefels, Barcelona, Spain
⁷ Centro de Astrobiología (CSIC-INTA), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁰ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹² Departamento de Física de la Tierra y Astrofísica & IPARCOS Instituto de Física de Partículas y del Cosmos, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Plaza de Ciencias 1, 28400 Madrid, Spain
¹³ Centro Astronómico Hispano en Andalucía, Observatorio Astronómico de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹⁴ Department of Astronomy/Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁵ Centre for Planetary Habitability, Department of Geosciences, Universitetet i Oslo, Sem Sælands vei 2b, 0315 Oslo, Norway
¹⁶ Department of Astronomy, Sofia University “St Kliment Ohridski”, 5 James Bourchier Blvd, 1164 Sofia, Bulgaria

^★ Corresponding author; a.kaminski@lsw.uni-heidelberg.de

Received: 10 December 2024
Accepted: 13 March 2025

Abstract

Aims. Previous estimates of planet occurrence rates in the CARMENES survey indicated increased numbers of planets on short orbits for M dwarfs with masses below 0.34M_⊙. Here we focused on the lowest-mass stars in the survey, comprising 15 inactive targets with masses under 0.16 M_⊙.

Methods. To correct for detection biases, we determined detection sensitivity maps for individual targets and the entire sample. Using Monte Carlo simulations, we estimated planet occurrence rates for orbital periods of 1 d to 100 d and minimum masses from 0.5 M_⊕ to 10M_⊕. We also compared the actual sample of known planets to model predictions.

Results. The radial velocity (RV) data from CARMENES reveal four new planets around three stars in our sample, namely G 268– 110 b, G 261–6 b, and G 192–15 b and c. All three b planets have minimum masses of 1.03–1.52 M_⊕ and orbital periods of 1.43–5.45 d, while G 192–15 c is a 14.3 M_⊕ planet on a wide, eccentric orbit with P ≈ 1218 d and e ≈ 0.68. Our occurrence rates suggest considerable dependencies with respect to stellar masses. For planets below 3 M_⊕ we found rates consistent with one planet per star across all investigated periods, but the rates decrease almost by an order of magnitude for larger planet masses up to 10 M_⊕. Compared to previous studies, low-mass stars tend to harbor more planets with P < 10 d. We also demonstrate that synthetic planet populations based on the standard core accretion scenario predict slightly more massive planets on wider orbits than observed.

Conclusions. Our findings confirm that planet occurrence rates vary with stellar masses even among M dwarfs, as we found more planets with lower masses and on shorter orbits in our subsample of very low-mass stars compared to more massive M dwarfs. Therefore, we emphasize the need for additional differentiation in future studies.