Deep inside a sub-Neptune, where pressures reach a hundred thousand bars and temperatures hit four thousand degrees, rock behaves like boiling soup. Heavy vapors of silicon and magnesium rise, cool, and fall back as a rain of solid particles. This cycle is not just a detail: it is the conductor directing the entire chemical orchestra of the atmosphere.
Earlier models pictured a sub-Neptune as a layered cake: a silicate core, a hydrogen envelope on top, and a nearly impermeable boundary between them. But calculations show that at such temperatures and pressures, the molten core actively exchanges material with the gas envelope. Gases dissolve into the magma, and rock vapors escape upward. To ignore this exchange is to describe not a planet but a caricature.
The result is as sharp as a switch. If the magma has enough oxygen, silicon and magnesium quickly bind into solid minerals and precipitate, leaving the upper atmosphere rich in water and carbon monoxide. Such an atmosphere is heavier than a hydrogen one, and the planet shrinks: its radius drops by twenty percent. If oxygen is scarce, the condensates take it almost entirely, leaving light methane and silane above. The radius shrinks by only ten percent. The transition between these two worlds is not gradual but abrupt, at a specific ratio of magnesium, silicon, and oxygen in the melt.
For observers, this means that two planets with the same mass and the same amount of hydrogen can have radii differing by twenty percent — simply because of how much oxygen was in their rocky interiors. JWST will be able to distinguish these types by the lines of water, methane, and silane in their spectra. The magnesium rain falling in the depths of distant worlds leaves a fingerprint visible hundreds of light-years away.
The model is named Rocky Raccoon after the Beatles song: it computes 'Rock vapor and Radial Atmospheric Composition', that is, vapor of rocky material and the radial distribution of atmospheric composition.
In Stanisław Lem's novel Solaris, the planet's ocean actively interacts with the atmosphere and even forms structures — much like a sub-Neptune's magma ocean, through evaporation and condensation of silicates, governs the chemistry and thermal regime of the envelope.