Sibling sub-Neptunes around sibling M dwarfs: TOI-521 and TOI-912

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 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁵ Department of Social Data Science, Hitotsubashi University, 2-1 Naka, Kunitachi, Tokyo 186-8601, Japan
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
⁷ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Barcelona, Spain
⁸ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁰ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
¹¹ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹² Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁴ Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁵ Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond St, London, Ontario, N6A 3K7, Canada
¹⁶ UTokyo Organization for Planetary and Space Science,The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
¹⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁸ Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita, Okayama 700-8530, Japan
¹⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²⁰ NASA Ames Research Center, Moffett Field, CA 94035, USA
²¹ Centre for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²² ETH Zurich, Department of Physics, Wolfgang-Pauli-Strasse 2, 8093 Zurich, Switzerland
²³ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
²⁴ Department of Physics & Astronomy, Texas Tech University, Lubbock TX, 79409-1051, USA
²⁵ Observatori Astronòmic Albanyà, Camí de Bassegoda S/N, Albanyà 17733, Girona, Spain
²⁶ Department of Multi-Disciplinary Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²⁷ Institute for Astronomy, University of Hawaii, 640 N. Aohoku Place, Hilo, HI 96720, USA
²⁸ SUPA Physics and Astronomy, University of St. Andrews, Fife, KY16 9SS Scotland, UK
²⁹ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, Japan
³⁰ SETI Institute, Mountain View, CA 94043, USA
³¹ Department of Physical Sciences, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan
³² Trottier Space Institute at McGill, McGill University, 3550 University Street, Montreal, QC H3A 2A7, Canada
³³ Subaru Telescope, National Astronomical Observatory of Japan, 650 N. Aohoku Place, Hilo, HI 96720, USA
³⁴ Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-26 Nakacho, Koganei, Tokyo, 184-8588, Japan
³⁵ South African Astronomical Observatory, PO Box 9, Observatory, Cape Town 7935, South Africa
³⁶ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³⁷ Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
³⁸ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

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Received: 18 September 2025
Accepted: 21 November 2025

Abstract

Context. Sub-Neptunes are absent in the Solar System, yet they are the most common category of planets found in our Galaxy. This kind of planet challenges the internal structure models, prompts investigations into its formation and evolution, and pushes atmospheric characterisation studies to break the degeneracy in their inner composition.

Aims. We report here the discovery and characterisation of new sub-Neptunes orbiting two similar M dwarfs, TOI-521 (T_eff = 3544 ± 100 K, V = 14.7 mag) and TOI-912 (T_eff = 3572 ± 100 K, V = 12.7 mag). Each star hosts a transiting planetary candidate identified by TESS and is part of the THIRSTEE follow-up programme, which aims to understand the sub-Neptune population through in-depth and precise characterisation studies on a population level.

Methods. We analysed TESS light curves, ground-based photometry, and high-precision ESPRESSO, HARPS, and IRD radial velocities to confirm the planetary nature of both candidates, infer the precise orbital and physical parameters of the planets, and investigate the presence of additional planets in the systems.

Results. The two stars host nearly identical planets in terms of mass and radius. TOI-521 hosts a transiting sub-Neptune in a 1.5-day orbit with radius and mass of R_b = 1.98 ± 0.14 R_⊕ and M_b = 5.3 ± 1.0 M_⊕, respectively. Moreover, we identified an additional candidate at 20.3 days, with a minimum mass of M_p sin i = 10.7_−2.4^+2.5 M_⊕, currently not detected as transiting in our photometric dataset. Similarly, the planet orbiting TOI-912 is a 4.7-d sub-Neptune with R_b = 1.93 ± 0.13 R_⊕ and M_b = 5.1 ± 0.5 M_⊕. Interestingly, TOI-912 b likely possesses an unusually high eccentricity (e = 0.58 ± 0.02) and is probably undergoing strong tidal dissipation. If such eccentricity were confirmed, it would make TOI-912 b one of the most eccentric sub-Neptunes known to date. TOI-521 b and TOI-912 b have very similar densities (~4 g cm⁻³), and they lie in the degenerate region of the mass-radius diagram where different compositions are plausible, including a volatile-rich composition, or a rocky core surrounded by a H-He envelope. When compared to the other THIRSTEE M-dwarf targets, our sample supports the division of sub-Neptunes into two distinct populations divided by a density gap. Both planets are interesting targets for atmospheric follow-up in the context of understanding the temperature-atmospheric feature trend that starts to emerge thanks to JWST observations.