Characterization of two new transiting sub-Neptunes and a terrestrial planet around M-dwarf hosts

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 Department of Physics, Aristotle University of Thessaloniki, University Campus, Thessaloniki, 54124, Greece
⁴ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, Napoli, Italy
⁶ INFN, Sezione di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia Edificio 6, 80126, Napoli, Italy
⁷ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131, Napoli, Italy
⁸ Centro de Astrobiología (CSIC-INTA), European Space Astronomy Centre, Camino Bajo del Castillo, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ Instituto de Astrofísica de Andalucía (IAA), 18080 Granada, Spain
¹⁰ INAF, Osservatorio Astronomico di Palermo, 90134 Palermo, Italy
¹¹ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
¹² Institut d’Estudis Espacials de Catalunya (IEEC), C/Esteve Terrades 1, Edifici RDIT, 08860 Castelldefels, Spain
¹³ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Barcelona, Spain
¹⁴ NASA Exoplanet Science Institute – Caltech/IPAC 1200 E. California Blvd Pasadena, CA 91125, USA
¹⁵ Center for Astrophysics I Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁶ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
¹⁷ Vereniging Voor Sterrenkunde, Oude Bleken 12, 2400 Mol, Belgium
¹⁸ AstroLAB IRIS, Provinciaal Domein “De Palingbeek”, Verbrandemolenstraat 5, 8902 Zillebeke, Ieper, Belgium
¹⁹ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²⁰ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²¹ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07775 Tautenburg, Germany
²³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁴ SUPA Physics and Astronomy, University of St. Andrews, Fife, KY16 9SS, Scotland, UK
²⁵ NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁶ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
²⁷ Agrupació Astronomica Sabadell, Carrer Prat de la Riba, 116, 08206 Sabadell, Barcelona, Spain
²⁸ Department of Physics and Astronomy, The University of New Mexico, 210 Yale Blvd. NE., Albuquerque, NM 87106, USA
²⁹ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁰ Department of Physics, Ariel University, Ariel 40700, Israel
³¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, 69117 Heidelberg, Germany
³² Institut für Astrophysik und Geophysik Georg-August-Universität, Friedrich Hund Platz 1, 37077 Göttingen, Germany
³³ South African Astronomical Observatory, PO Box 9, Observatory, Cape Town 7935, South Africa
³⁴ Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
³⁵ Astrophysics, Geophysics, And Space Science Research Center, Ariel University, Ariel 40700, Israel
³⁶ Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
³⁷ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

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Received: 8 September 2025
Accepted: 21 December 2025

Abstract

We report the confirmation of three transiting exoplanets orbiting TOI-1243 (LSPM J0902+7138), TOI-4529 (G 2-21), and TOI-5388 (Wolf 346) that were initially detected by TESS through ground-based photometry and radial velocity follow-up measurements with CARMENES. The planets present short orbital periods of 4.65, 5.88, and 2.59 days, and they orbit early-M dwarfs (M2.0 V, M1.5 V, and M3.0 V, respectively). We were able to precisely determine the radius of all three planets with a precision of <7%, the mass of TOI-1243 b with a precision of 19%, and upper mass limits for TOI-4529 b and TOI-5388 b. The radius of TOI-1243 b is 2.33 ± 0.12 R_⊕, its mass is 7.7 ± 1.5 M_⊕, and the mean density is 0.61 ± 0.15 ρ_⊕. The radius of TOI-4529b is 1.77_−0.08^+0.09 R_⊕, the 3σ upper mass limit is 4.9 M_⊕, and the 3σ upper density limit is 0.88 ρ_⊕. The third planet, TOI-5388 b, is Earth-sized with a radius of 0.99_−0.06^+0.07 R_⊕, a 3σ upper mass limit of 2.2 M_⊕, and a 3σ upper density limit of 2.2 ρ_⊕. While TOI-5388 b is most probably rocky, given its Earth-like radius, TOI-1243 b and TOI-4529 b are located in a highly degenerate region in the mass-radius space. TOI-4529 b appears to lean toward a water-world composition. TOI-1243 b has enough mass to host a significant H-He envelope, although a water-world and pure rocky compositions are also consistent with the data. Our analysis indicates that future atmospheric observations using JWST can aid in determining their real composition. The sample of small planets around M dwarfs is widely used to understand planet formation and composition theories, and our study adds three planets to this sample.