Inside a sub-Neptune, rock boils like soup in a pot. Vapors of silicon and magnesium rise, cool, and fall back as rain of solid sand grains. This rain acts like a conductor: it decides which gases stay in the atmosphere and which sink deeper.
Previously, scientists thought that the rocky core and the hydrogen envelope live separately, like yolk and white in an egg. But at 4000 degrees and a pressure of a hundred thousand atmospheres, the boundary between them blurs. Gases penetrate the molten rock, and rock vapors escape upward. The new model is the first to account for this exchange and shows what it leads to.
This explains a long-standing puzzle: why sub-Neptunes of the same mass can have noticeably different sizes. The answer lies in the composition of their rocky interiors. The JWST telescope can distinguish such planets by their spectra, seeing lines of water, methane, and silane. The rocky rain in the depths of distant worlds leaves a trace visible from 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.