Not All Sub-Neptune Exoplanets Have Magma Oceans
The evolution and structure of sub-Neptunes may be strongly influenced by interactions between the outer gaseous envelope of the planet and a surface magma ocean.
However, given the wide variety of permissible interior structures of these planets, it is unclear whether conditions at the envelope-mantle boundary will always permit a molten silicate layer, or whether some sub-Neptunes might instead host a solid silicate surface. In this work, we use internal structure modeling to perform an extensive exploration of surface conditions within the sub-Neptune population across a range of bulk and atmospheric parameters.
We find that a significant portion of the population may lack present-day magma oceans. In particular, planets with a high atmospheric mean molecular weight and large envelope mass fraction are likely to instead have a solid silicate surface, since the pressure at the envelope-mantle boundary is high enough that the silicates will be in solid post-perovskite phase.
This result is particularly relevant given recent inferences of high-mean molecular weight atmospheres from JWST observations of several sub-Neptunes. We apply this approach to a number of sub-Neptunes with existing or upcoming JWST observations, and find that in almost all cases, a range of solutions exist which do not possess a present-day magma ocean. Our analysis provides critical context for interpreting sub-Neptunes and their atmospheres.
Bodie Breza, Matthew C. Nixon, Eliza M.-R. Kempton
Comments: 15 pages, 4 figures, 2 tables. Re-submitted to AAS journals after addressing reviewer comments
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.20429 [astro-ph.EP] (or arXiv:2509.20429v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.20429
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Submission history
From: Matthew Nixon
[v1] Wed, 24 Sep 2025 18:00:13 UTC (804 KB)
https://arxiv.org/abs/2509.20429
Astrobiology
