![D.2: Transit signals recovery analysis in the TESS light curve of TOI-210. TOI-210 b is shown in gold at its period of 9.01 days. Candidate planets at 2.15 and 3.76 days tentatively detected in the NIRPS RVs are indicated as lower limits, with arrows extending from minimum (iron density) to realistic (Earth density) radii based on their minimum masses. The expected transit depths fall within the TESS sensitivity, suggesting that if these RV signals are planets, they are unlikely to be transiting. -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/10/A-temperate.png?fit=1200%2C669&ssl=1)
[astro-ph.EP] Super-Earths and sub-Neptunes dominate the exoplanet population, yet their compositions, formation mechanisms, and capability of retaining atmospheres remain poorly understood. M-type stars offer a unique opportunity to investigate these questions, thanks to their favorable planet-to-star radius and mass ratios and the diversity of planetary systems they host.
We aim to characterise the planetary system of the M4 dwarf TOI-210, in which TESS during its primary mission identified one transiting exoplanet candidate at P=9.01 days. We combine multi-technique observations, including transits from 40 sectors of TESS and ground-based follow-up with LCOGT and ExTrA, and radial velocities (RV) from NIRPS obtained as part of its Guaranteed Time Observations program.
These data were analysed using a Bayesian framework to constrain stellar, planetary, and orbital parameters. We confirm TOI-210 b, a temperate sub-Neptune (Teq=368±9 K) with a mass of 6.75 ± 1.25 M⊕ and a radius of 2.234 ± 0.074 R⊕.
Its bulk density implies a volatile-rich composition, either as an extended H/He atmosphere with mass fraction of approximately 1% (gas dwarf), a substantial water reservoir comprising at least 29% of the mass (2σ lower limit; water world), or a mixture of both.
The NIRPS RVs show moderate evidence (ΔlnZ=5.9) for at least one non-transiting exoplanet interior to TOI-210 b. In a targeted search, we identify two Keplerian signals at 2.15 and 3.76 days, with minimum masses of 3.2 ± 0.8 M⊕ and 4.4 ± 1.0 M⊕, respectively, that provide a plausible explanation for the observed RV variations.
If these signals are planets, the absence of transits in the TESS photometry implies mutual inclinations with TOI-210 b above 2.3-3.5∘.
C. Cadieux, B. N. Skinner, F. Bouchy, E. D. Gillis, V. Bourrier, R. Doyon, L. Y. Messamah, B. L. Canto Martins, A. L’Heureux, Y. S. Messias, J. M. Almenara, A. K. Stefanov, X. Bonfils, K. A. Collins, H. M. Relles, C. Rodríguez, P.-A. Roy, A. Shporer, G. Srdoc, S. Taylor, C. Ziegler, R. Allart, K. Al Moulla, É. Artigau, F. Baron, S. C. C. Barros, F. Bélanger, B. Benneke, M. Bryan, T. Ciolak, R. Cloutier, N. J. Cook, N. B. Cowan, E. Cristo, L. Dang, J. R. De Medeiros, X. Delfosse, E. Delgado-Mena, X. Dumusque, D. Ehrenreich, D. O. Fontinele, T. Forveille, J. Gagné, M. J. Hobson, P. Lamontagne, I. C. Leão, R. de Lima Gomes, C. Lovis, L. Malo, C. Melo, L. Mignon, C. Mordasini, N. Nari, L. Parc, R. Rebolo, R. Rosener, J. Rowe, N. C. Santos, D. Ségransan, A. Srivastava, A. Suárez Mascareño, S. Udry, D. Valencia, G. Wade, J. P. Wardenier, D. Weisserman
Comments: 23 pages, 15 figures, and 7 tables. Submitted to A&A; currently under minor revision
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2609.35747 [astro-ph.EP] (or arXiv:2609.35747v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.35747
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Submission history
From: Charles Cadieux
[v1] Mon, 28 Sep 2026 17:56:20 UTC (12,829 KB)
https://arxiv.org/abs/2609.35747
Astrobiology, Exoplanet,
