The Surface and Interior Conditions of Temperate Sub-Neptune TOI-270 d

Sub-Neptune planets, with no analogue in our solar system, provide a wealth of information about exoplanet diversity, formation & evolution, and habitability.

Their robust characterisation requires the coupling of physically informed atmosphere and interior models with precise atmospheric data to break compositional degeneracies. Recent JWST observations of the temperate sub-Neptune TOI-270 d revealed detections of CH₄ and CO₂ in its H₂-rich atmosphere, with tentative inferences of H₂O and CS₂ and a non-detection of NH3. We conduct a theoretical exploration of the range of possible interiors for TOI-270 d based on the current observational constraints.

We carry out internal structure modelling using a coupled atmosphere-interior model, including self-consistent atmospheric temperature structures informed by JWST observations. The bulk properties permit solutions spanning mini-Neptune, gas dwarf and hycean scenarios, with a wide range of possible surface conditions, which are strongly dependent on the atmospheric properties, including the presence of clouds/hazes.

We explore the solutions allowing for surface water oceans on TOI-270 d, including under potentially habitable conditions. The atmospheric mass fractions permitting habitable surface conditions are found to be ≲3.5×10⁻⁵ and pressures ≲100 bar for the envelope temperature structures considered. We consider mini-Neptune interiors that are sufficiently warm for H₂O to be mixed with the H2-rich envelope.

Finally, we consider possible gas dwarf interiors, finding H₂-rich envelope mass fractions of ∼1−5 % are required to satisfy the bulk properties, with surface pressures ∼10⁴−10⁵ bar. Further theoretical and experimental studies in addition to future atmospheric observations will aid the characterisation of the possible interior and surface conditions on TOI-270 d.

Frances E. Rigby, Nikku Madhusudhan

Comments: 14 pages, 9 figures. Accepted to MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.16722 [astro-ph.EP] (or arXiv:2511.16722v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.16722
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Submission history
From: Frances Rigby
[v1] Thu, 20 Nov 2025 19:00:00 UTC (995 KB)
https://arxiv.org/abs/2511.16722
Astrobiology,