2026A&A...705A..25C

The first explorations of temperate sub-Neptune exoplanets have been the hallmark of early JWST observations. The bulk properties of such planets are consistent with a range of possible internal structures, which can be distinguished through their interactions with the observable atmospheres. JWST observations of TOI-270 d, a temperate sub-Neptune, have previously led to contrasting conclusions: either a Hycean world, possessing a liquid water ocean, or a mixed-envelope sub-Neptune, where high temperatures prevent a liquid ocean and lead to a high mean molecular weight atmosphere. In order to resolve this uncertainty, we present a comprehensive retrieval analysis of TOI-270 d using recent NIRISS and NIRSpec transit spectroscopy across ∼1-5 µm. We find that prior inferences of a mixed envelope were affected by specific modelling choices leading to a high terminator temperature and high mean-molecular weight in the atmosphere. We confirm an H₂-rich atmosphere in TOI-270 d and present revised constraints on the molecular log-mixing ratios and maximal detection significances of -1.86_–0.29^+0.30 (6.4σ) CO₂ at -1.71_–0.66^+0.38 (3.9σ), H₂O at .-1.88_–4.13^+0.78(2.1σ) and CS₂ at -4.74_–1.10^+0.65(2.0σ), with a terminator temperature of 323_–52⁺⁵⁸ K at 10 mbar.We also find tentative evidence for more complex methyl-bearing species such as C2H6 and/or DMS at a 2.1-2.5 σ level. The present constraints are consistent with TOI-270 d being a Hycean or dark Hycean world, with planet-wide or nightside liquid water oceans. However, more observations are required to verify the present findings and robustly constrain the atmospheric conditions and internal structure of TOI-270 d.