A temperate sub-Neptune transiting the M4 Dwarf TOI-210 identified by NIRPS and TESS -- Uncovering hidden M-dwarf planetary systems in the near-infrared

@misc{cadieux2026temperate,
  title = {{A temperate sub-Neptune transiting the M4 Dwarf TOI-210 identified by NIRPS and TESS -- Uncovering hidden M-dwarf planetary systems in the near-infrared}},
  author = {C. Cadieux and B. N. Skinner and F. Bouchy and E. D. Gillis and V. Bourrier and R. Doyon and L. Y. Messamah and B. L. Canto Martins and A. L'Heureux and Y. S. Messias and J. M. Almenara and A. K. Stefanov and X. Bonfils and K. A. Collins and H. M. Relles and C. Rodríguez and P. -A. Roy and A. Shporer and G. Srdoc and S. Taylor and C. Ziegler and R. Allart and K. Al Moulla and É. Artigau and F. Baron and S. C. C. Barros and F. Bélanger and B. Benneke and M. Bryan and T. Ciolak and R. Cloutier and N. J. Cook and N. B. Cowan and E. Cristo and L. Dang and J. R. De Medeiros and X. Delfosse and E. Delgado-Mena and X. Dumusque and D. Ehrenreich and D. O. Fontinele and T. Forveille and J. Gagné and M. J. Hobson and P. Lamontagne and I. C. Leão and R. de Lima Gomes and C. Lovis and L. Malo and C. Melo and L. Mignon and C. Mordasini and N. Nari and L. Parc and R. Rebolo and R. Rosener and J. Rowe and N. C. Santos and D. Ségransan and A. Srivastava},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.35747},
  eprint = {2609.35747},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.35747},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - A temperate sub-Neptune transiting the M4 Dwarf TOI-210 identified by NIRPS and TESS -- Uncovering hidden M-dwarf planetary systems in the near-infrared
AU  - C. Cadieux
AU  - B. N. Skinner
AU  - F. Bouchy
AU  - E. D. Gillis
AU  - V. Bourrier
AU  - R. Doyon
AU  - L. Y. Messamah
AU  - B. L. Canto Martins
AU  - A. L'Heureux
AU  - Y. S. Messias
AU  - J. M. Almenara
AU  - A. K. Stefanov
AU  - X. Bonfils
AU  - K. A. Collins
AU  - H. M. Relles
AU  - C. Rodríguez
AU  - P. -A. Roy
AU  - A. Shporer
AU  - G. Srdoc
AU  - S. Taylor
AU  - C. Ziegler
AU  - R. Allart
AU  - K. Al Moulla
AU  - É. Artigau
AU  - F. Baron
AU  - S. C. C. Barros
AU  - F. Bélanger
AU  - B. Benneke
AU  - M. Bryan
AU  - T. Ciolak
AU  - R. Cloutier
AU  - N. J. Cook
AU  - N. B. Cowan
AU  - E. Cristo
AU  - L. Dang
AU  - J. R. De Medeiros
AU  - X. Delfosse
AU  - E. Delgado-Mena
AU  - X. Dumusque
AU  - D. Ehrenreich
AU  - D. O. Fontinele
AU  - T. Forveille
AU  - J. Gagné
AU  - M. J. Hobson
AU  - P. Lamontagne
AU  - I. C. Leão
AU  - R. de Lima Gomes
AU  - C. Lovis
AU  - L. Malo
AU  - C. Melo
AU  - L. Mignon
AU  - C. Mordasini
AU  - N. Nari
AU  - L. Parc
AU  - R. Rebolo
AU  - R. Rosener
AU  - J. Rowe
AU  - N. C. Santos
AU  - D. Ségransan
AU  - A. Srivastava
PY  - 2026
DA  - 2026/09/28
PB  - arXiv
DO  - 10.48550/arxiv.2609.35747
UR  - https://arxiv.org/abs/2609.35747
AB  - 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 (T_eq = 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 (Δln Z = 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^∘.
ER  -