A Window For Water-hydrogen Demixing On Warm Metal-rich Sub-Neptunes

Sub-Neptunes represent the largest exoplanet demographic, yet their bulk compositions remain poorly understood.

Recent studies suggested that only very cold planets, such as Uranus and Neptune, could experience stratification of volatiles in their envelopes, implying that the envelopes of warmer sub-Neptunes instead have fully-miscible compositions.

Here, we present ATHENAIA, an interior-atmosphere composition inference framework we leverage to assess the potential for water-hydrogen demixing on the Teq=350 K planet TOI-270 d, and more broadly for warm sub-Neptunes, using radiative-convective atmosphere models coupled to interior models.

We find that the higher temperatures at which hydrogen and water demix in water-rich environments, combined with the shallower adiabatic gradients of water-rich envelopes, open a window for demixing on sub-Neptunes with bulk envelope metallicities of ∼100 to 700× solar, compatible with TOI-270 d. Demixing is easier to achieve on more massive and colder planets, but still broadly affects warm (330 to 500 K) metal-rich sub-Neptunes.

Therefore, combining atmosphere metallicities with models of fully-miscible envelopes may lead to underestimated bulk envelope metallicities and mass fractions. Further, our modeling of TOI-270 d’s envelope and interior reveals that, for a typical internal energy budget Tint of 25 K, the envelope-mantle boundary conditions likely preclude the presence of a molten magma ocean.

This work encourages a reconsideration of the current paradigm for linking sub-Neptune atmospheres to their interiors and motivates further evolutionary modeling describing the onset of metallicity gradients in sub-Neptune envelopes.

Caroline Piaulet-Ghorayeb, Daniel P. Thorngren, Eliza M.-R. Kempton, Justin Lipper, Leslie Rogers, Fernanda Correa Horta, Shi Lin Sun

Comments: 19 pages, 10 figures, 2 tables, in review at AAS Journals
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2512.01805 [astro-ph.EP] (or arXiv:2512.01805v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.01805
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Submission history
From: Caroline Piaulet-Ghorayeb
[v1] Mon, 1 Dec 2025 15:39:33 UTC (2,674 KB)
https://arxiv.org/abs/2512.01805

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