Most Rocky Sub-Neptunes are Molten: Mapping the Solidification Shoreline for Gas Dwarf Exoplanets
23 Apr 2026, Yanjiang
Every day, telescopes across the world add to the growing catalog of exoplanets. Every day, those same telescopes reveal worlds so different from anything in our solar system that we’re still struggling to understand what they’re made of. The most common type of exoplanet we’ve discovered — the sub-Neptune — remains one of the most mysterious. Are they mini-Neptunes with thick atmospheres? Are they rocky worlds with puffed-up hydrogen envelopes? Or are they something else entirely?
Now, a team led by Claire-Marie Guimond at the University of Cambridge has proposed a surprising answer. Their work, appearing in a preprint (arXiv:2512.05816), suggests that most sub-Neptunes — if they are what the team calls “gas dwarfs” — are not cold, solid worlds at all. They are molten. Permanently, globally molten, with magma oceans that have never frozen.
The result changes how we think about the most abundant type of planet in the galaxy.
The Gas Dwarf Hypothesis
To understand why this matters, we first need to understand what a gas dwarf is. Imagine a rocky planet — something like Earth, but larger. Now wrap it in a thin atmosphere of hydrogen and helium. The atmosphere is substantial enough to give the planet a puffy appearance, but the bulk of the mass is still in the rocky interior. That’s a gas dwarf: a silicate/iron core with a hydrogen-dominated atmosphere.
This is one of several competing models for sub-Neptunes. The problem is that when astronomers measure a sub-Neptune’s mass and radius, multiple interior structures can produce the same numbers. A planet might be a water world with a thick steam atmosphere. It might be a rocky core with a deep ocean. Or it might be a gas dwarf. The data alone can’t tell them apart.
But gas dwarfs, if they exist, have one feature that might break the degeneracy: their interiors should interact with their atmospheres in ways that produce detectable signatures. A solid interior sits quietly beneath its atmosphere. A molten interior, by contrast, constantly exchanges material with the overlying gases — rock vapor mixing with hydrogen, oxygen dissolving into magma. These interactions leave chemical fingerprints that future telescopes might detect.
The catch is that this only works if the planet is actually molten. And that depends on how much energy it receives from its star.
The Solidification Shoreline
Guimond and colleagues — Robb Calder, Oliver Shorttle, Harrison Nicholls, and Tim Lichtenberg — used a coupled interior-climate evolution model called PROTEUS to map out the conditions under which gas dwarfs remain molten versus solidifying. Think of it like a coastline on a map: on one side, planets are hot enough to keep their interiors liquid; on the other, they’ve cooled enough to freeze. The team calls this boundary the “solidification shoreline.”
The key variable is instellation flux — the amount of stellar radiation the planet receives per unit area. This is not the same as the star’s total brightness. It depends on both the star’s temperature and the planet’s orbital distance. A planet close to a hot star gets blasted with radiation; a planet far from a cool star receives barely a trickle.
The results were striking. When the team ran their models across the full range of known sub-Neptunes — planets with radii between 1.5 and 4 times Earth’s, orbiting stars of all temperatures — they found that 98% of detected sub-Neptunes occupy a region of parameter space consistent with having permanent magma oceans, if they are gas dwarfs.
Let that sink in. Almost every sub-Neptune we’ve ever found, if it’s a gas dwarf, is molten today. Not recently molten. Not partially molten. Permanently, globally molten, with magma oceans that have persisted for billions of years.
What Keeps Them Hot?
The reason is surprisingly simple. Sub-Neptunes are born hot — the accretion process that forms them leaves their interiors molten. The question is whether they cool down fast enough to solidify over the age of the galaxy. For most sub-Neptunes, the answer appears to be no.
The team’s models show that the solidification shoreline depends on two additional factors beyond instellation flux: the mantle’s oxidation state (how much oxygen is available in the rock) and the bulk volatile carbon-to-hydrogen ratio. These factors influence how efficiently the planet can radiate heat into space. A planet with an oxidizing mantle and carbon-rich atmosphere, for example, might develop a high mean-molecular-weight atmosphere that traps heat more effectively, keeping the interior molten even at lower instellation fluxes.
But these cases are outside the scope of the current study. For the simpler case — planets with hydrogen-dominated atmospheres and moderate oxidation states — the picture is clear: most detected sub-Neptunes lie well above the solidification shoreline.
Why This Matters
This result has immediate implications for both theory and observation.
Theoretically, it means that if sub-Neptunes are gas dwarfs, their interiors are not static. Magma oceans are dynamic systems. They convect. They outgas. They react with overlying atmospheres. Understanding how these interactions evolve over time — and how they affect the planet’s long-term evolution — becomes essential for interpreting any observations we make.
Observationally, it means that the chemical signatures of magma-atmosphere interactions should be present in the atmospheres of most sub-Neptunes, if they are gas dwarfs. Future missions like the James Webb Space Telescope, or next-generation ground-based spectrographs, can look for these signatures. Species like silicon monoxide, sodium, or potassium in the upper atmosphere would be telltale signs of a molten interior actively exchanging material with its sky.
If these signatures are found, it would not only confirm the gas dwarf model but also open a new window into planetary interiors. We could study the composition of magma oceans on other worlds — worlds we will never visit but whose chemistry we can read from light.
The Road Ahead
The team is careful to note that their results apply only to the gas dwarf interpretation of sub-Neptunes. Other models — water worlds, mini-Neptunes with deep H/He envelopes — remain viable. But the solidification shoreline provides a clear prediction: if you observe a sub-Neptune and find evidence for ongoing magma-atmosphere interaction, you’ve likely confirmed the gas dwarf model.
For a field that has long struggled to distinguish between competing interior models, this is a significant step forward. The next generation of exoplanet atmosphere observations will have a specific target to aim for: the chemical signatures of a planet that has never cooled down, whose surface is a global ocean of molten rock.
And that, perhaps, is the most remarkable implication of all. The most common type of planet in the galaxy may not be a cold, quiet world like our own. It may be a world of fire — a world where the distinction between rock and atmosphere blurs, where the surface is a seething ocean that has churned for billions of years, and where the boundary between solid and liquid is not a line on a graph but a shoreline that almost no planet has crossed.
Yanjiang is an online editor of Loom Science
References
- R. Calder et al., Most Rocky Sub-Neptunes are Molten: Mapping the Solidification Shoreline for Gas Dwarf Exoplanets, arXiv:2512.05816