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arXiv · 2609.37597

TOI-6981 b: a transiting sub-Neptune at the water ice line

Madison G. Scott·Amaury H. M. J Triaud·Emily K. Pass·Aleyna Adamson·Karen A. Collins·Khalid Barkaoui·Allyson Bieryla·Adam J. Burgasser·Mark E. Everett·Steve B. Howell·David W. Latham·Benjamin V. Rackham·Boris S. Safonov·Miquel Serra-Ricart·Abderahmane Soubkiou·Georgina Dransfield·Katharine Hesse·Pipa Fernandez de Diego·Emma Softich·Gregor Srdoc·Mathilde Timmermans·Jeroen Audenaert·Roi Alonso·Artem Y. Burdanov·Yasmin T. Davis·Fatemeh Davoudi·Julien de Wit·Shishir Dholakia·Elsa Ducrot·Michaël Gillon·Yilen Gomez Maqueo Chew·Matthew J. Hooton·Clàudia Janó-Muñoz·Emmanuel Jehin·Akanksha Khandelwal·Thomas R. Marriott·Peter P. Pedersen·Manuel Pichardo Marcano·Francisco J. Pozuelos·Rafael Rebolo-López·Manu Stalport·Alexander Venner·Selçuk Yalçinkaya·Francis Zong Lang·Sebastián Zúñiga-Fernández
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Abstract

We present TOI-6981 b, a cold (Teq= 170.0 +/-4.5 K, assuming an Earth-like albedo) sub-Neptune (Rb=2.40+/-0.10 R_oplus) on a 60.8 day orbit around around its M4-type host star (R*=0.304 +/- 0.012 R_odot, M*=0.296 +/- 0.026 M_odot, Teff = 3215+/-58K), part of the SPECULOOS TEMPOS program. We validate the planetary nature of TOI-6981 b using ground-based follow-up photometry, statistical validation and high-resolution spectroscopy. At such cold temperatures, TOI-6981 b sits roughly at the location of the water ice line (approx 170 K). Given its orbital period and high transmission spectroscopy metric (TSM=51 +/- 9), it is one of the only planets in the sparsely populated cold planet regime amenable for atmospheric studies with JWST. Therefore, TOI-6981 b could. possibly become a crucial benchmark in cold chemistry studies, tests of Hycean world hypotheses, and drawling parallels to the Solar System's ice giants. Additionally, given its current orbit, it may provide insight into planetary formation and migration at or beyond the water ice line.

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Madison G. Scott, Amaury H. M. J Triaud, Emily K. Pass, Aleyna Adamson, Karen A. Collins, Khalid Barkaoui, Allyson Bieryla, Adam J. Burgasser, Mark E. Everett, Steve B. Howell, David W. Latham, Benjamin V. Rackham, Boris S. Safonov, Miquel Serra-Ricart, Abderahmane Soubkiou, Georgina Dransfield, Katharine Hesse, Pipa Fernandez de Diego, Emma Softich, Gregor Srdoc, Mathilde Timmermans, Jeroen Audenaert, Roi Alonso, Artem Y. Burdanov, Yasmin T. Davis, Fatemeh Davoudi, Julien de Wit, Shishir Dholakia, Elsa Ducrot, Michaël Gillon, Yilen Gomez Maqueo Chew, Matthew J. Hooton, Clàudia Janó-Muñoz, Emmanuel Jehin, Akanksha Khandelwal, Thomas R. Marriott, Peter P. Pedersen, Manuel Pichardo Marcano, Francisco J. Pozuelos, Rafael Rebolo-López, Manu Stalport, Alexander Venner, Selçuk Yalçinkaya, Francis Zong Lang, Sebastián Zúñiga-Fernández. 2026-09-29. TOI-6981 b: a transiting sub-Neptune at the water ice line. https://arxiv.org/abs/2609.37597

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