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 $\sim$1-5 $\mu$m. 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 H$_2$-rich atmosphere in TOI-270 d and present revised constraints on the molecular log-mixing ratios and maximal detection significances of CH$_4$ at $-1.86^{ +0.30}_{ -0.29}$ (6.4 $\sigma$), CO$_2$ at $-1.71^{ +0.38}_{ -0.66}$ (3.9 $\sigma$), H$_2$O at $-1.88^{ +0.78}_{ -4.13}$ (2.1 $\sigma$) and CS$_2$ at $-4.74^{ +0.65}_{ -1.10}$ (2.0 $\sigma$), with a terminator temperature of $323^{ +58}_{ -52}$ K at 10 mbar. We also find tentative evidence for more complex methyl-bearing species such as C$_2$H$_6$ and/or DMS at a 2.1-2.5 $\sigma$ 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.
Categories: astro-ph.EP