The Atmospheric Composition Of TOI-270 d

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 CH₄ at −1.86+0.30−0.29 (6.4 σ), CO₂ at −1.71+0.38−0.66 (3.9 σ), H₂O at −1.88+0.78−4.13 (2.1 σ) and CS₂ at −4.74+0.65−1.10 (2.0 σ), 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 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.

Savvas Constantinou, Nikku Madhusudhan, Måns Holmberg

Comments: Accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.13830 [astro-ph.EP] (or arXiv:2511.13830v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.13830
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Submission history
From: Madhusudhan Nikku
[v1] Mon, 17 Nov 2025 19:00:05 UTC (30,528 KB)
https://arxiv.org/abs/2511.13830
Astrobiology,