{
  "abstract": "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 ($T_{\\rm eq} = 368 \\pm 9$ K) with a mass of 6.75 $\\pm$ 1.25 M$_{\\oplus}$ and a radius of 2.234 $\\pm$ 0.074 R$_{\\oplus}$. 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 (2$σ$ lower limit; water world), or a mixture of both. The NIRPS RVs show moderate evidence ($Δ\\ln Z = 5.9$) 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 $\\pm$ 0.8 M$_{\\oplus}$ and 4.4 $\\pm$ 1.0 M$_{\\oplus}$, 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$^{\\circ}$.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.35747",
  "author_total": 66,
  "authors": [
    {
      "name": "C. Cadieux"
    },
    {
      "name": "B. N. Skinner"
    },
    {
      "name": "F. Bouchy"
    },
    {
      "name": "E. D. Gillis"
    },
    {
      "name": "V. Bourrier"
    },
    {
      "name": "R. Doyon"
    },
    {
      "name": "L. Y. Messamah"
    },
    {
      "name": "B. L. Canto Martins"
    },
    {
      "name": "A. L'Heureux"
    },
    {
      "name": "Y. S. Messias"
    },
    {
      "name": "J. M. Almenara"
    },
    {
      "name": "A. K. Stefanov"
    },
    {
      "name": "X. Bonfils"
    },
    {
      "name": "K. A. Collins"
    },
    {
      "name": "H. M. Relles"
    },
    {
      "name": "C. Rodríguez"
    },
    {
      "name": "P. -A. Roy"
    },
    {
      "name": "A. Shporer"
    },
    {
      "name": "G. Srdoc"
    },
    {
      "name": "S. Taylor"
    },
    {
      "name": "C. Ziegler"
    },
    {
      "name": "R. Allart"
    },
    {
      "name": "K. Al Moulla"
    },
    {
      "name": "É. Artigau"
    },
    {
      "name": "F. Baron"
    },
    {
      "name": "S. C. C. Barros"
    },
    {
      "name": "F. Bélanger"
    },
    {
      "name": "B. Benneke"
    },
    {
      "name": "M. Bryan"
    },
    {
      "name": "T. Ciolak"
    },
    {
      "name": "R. Cloutier"
    },
    {
      "name": "N. J. Cook"
    },
    {
      "name": "N. B. Cowan"
    },
    {
      "name": "E. Cristo"
    },
    {
      "name": "L. Dang"
    },
    {
      "name": "J. R. De Medeiros"
    },
    {
      "name": "X. Delfosse"
    },
    {
      "name": "E. Delgado-Mena"
    },
    {
      "name": "X. Dumusque"
    },
    {
      "name": "D. Ehrenreich"
    },
    {
      "name": "D. O. Fontinele"
    },
    {
      "name": "T. Forveille"
    },
    {
      "name": "J. Gagné"
    },
    {
      "name": "M. J. Hobson"
    },
    {
      "name": "P. Lamontagne"
    },
    {
      "name": "I. C. Leão"
    },
    {
      "name": "R. de Lima Gomes"
    },
    {
      "name": "C. Lovis"
    },
    {
      "name": "L. Malo"
    },
    {
      "name": "C. Melo"
    },
    {
      "name": "L. Mignon"
    },
    {
      "name": "C. Mordasini"
    },
    {
      "name": "N. Nari"
    },
    {
      "name": "L. Parc"
    },
    {
      "name": "R. Rebolo"
    },
    {
      "name": "R. Rosener"
    },
    {
      "name": "J. Rowe"
    },
    {
      "name": "N. C. Santos"
    },
    {
      "name": "D. Ségransan"
    },
    {
      "name": "A. Srivastava"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/beztm1whmd9fe.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.35747",
  "fulltext_url": null,
  "id": "beztm1whmd9fe",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.35747",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.EP"
    ],
    [
      "openalex",
      "3103|Stellar, planetary, and galactic studies"
    ]
  ],
  "openalex": "W7214714068",
  "pdf_url": "https://arxiv.org/pdf/2609.35747",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-28",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/beztm1whmd9fe.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:12Z",
      "native_id": "2609.35747",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.35747"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.EP",
      "subfield": "planetary"
    }
  ],
  "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",
  "title_text": "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",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:12Z",
  "url": "https://research.draconitic.com/paper/beztm1whmd9fe",
  "venue": "arXiv",
  "versions": []
}