Water/Hycean Worlds & Oceanography

The Atmospheric Composition Of TOI-270 d

By Keith Cowing

Status Report

astro-ph.EP

November 22, 2025

Filed under astro-ph.EP, atmosphere, exoplanet, habitability, Hycean world, JWST, oceanworld, Spectroscopy, sub-Neptune, TOI-270 d

The Atmospheric Composition Of TOI-270 d — JWST transmission spectrum of TOI-270 d. Orange errorbars correspond to NIRISS observations, presented in this work, while dark red errorbars correspond to NIRSpec G395H observations presented by HM24. The NIRISS observations were binned down to R=25 for order 1 and R=12 for Order 2 for visual clarity. The blue curve denotes the median retrieved spectral fit obtained with our canonical retrieval, described in Section 3. Darker and lighter turquoise shaded regions denote the spectral 1- and 2-σ contours corresponding to the spectral fit. Yellow dots denote the median spectrum binned to the resolution of the datapoints. The NIRISS data are vertically offset by +28 ppm, which is the median offset retrieved for our canonical model. — astro-ph.EP

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,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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