Sub-Neptunes are the most common class of exoplanets, with radii between 2 and 4 Earth radii, yet their internal structure remains enigmatic. Classical models assumed a sharp division into a silicate core and a hydrogen envelope, but at temperatures above 2000 K and pressures above 10000 bar, silicates melt and gases dissolve readily in the melt. This means the atmosphere and interior exchange material, and envelope chemistry cannot be treated in isolation. Previous work either included chemistry without affecting structure or used simplified reaction networks that omitted magnesium and the diversity of condensates. This study fills that gap by combining, for the first time, the full set of elements Mg, Si, O, C, H with a self-consistent calculation of the thermal profile.
The authors built the Rocky Raccoon model, which couples the chemical equilibrium code FASTCHEM COND with an atmospheric structure model. The calculation starts at the base of the envelope, where a temperature of 4000 K and the composition of the magma ocean are specified, and steps upward in pressure. At each level, the equilibrium composition of gas and condensates is computed, with condensed solids removed from the gas phase, mimicking a rain of silicates. Simultaneously, the convective stability criterion is checked, accounting for the molecular weight gradient caused by condensation of heavy elements. The model iteratively adjusts the base pressure and heat flux to reproduce the specified hydrogen mass and equilibrium temperature, then builds profiles of temperature, density, and chemical composition up to a pressure of $10^{-3}$ bar.
Condensation of magnesium silicates radically changes the appearance of sub-Neptunes. Depending on the Mg:Si:O ratio in the melt, two extreme atmospheric types emerge. With excess oxygen (Mg:Si:O = 1:1:4), silicon and magnesium rapidly precipitate as MgSiO3, SiO2, and MgO, leaving an atmosphere rich in water and CO with a mean molecular weight of about 4 amu, and the planet's radius shrinks by 20% compared to a pure hydrogen envelope. With oxygen deficiency (Mg:Si:O = 1:1:3), oxygen is locked up in condensates, and the upper atmosphere becomes a mixture of methane and silane with a mean molecular weight of about 2 amu, while the radius contracts by only 10%. The transition between regimes is sharp and occurs at (Mg+Si)/O ≈ 0.67 and Mg/Si ≈ 1.3. Additionally, multiple non-convective layers form in the deep interior, making the planet more compact than adiabatic models predict.
This work shows that the chemical composition of sub-Neptune interiors directly controls their observable properties—radius and upper-atmosphere spectrum. This changes the interpretation of mass–radius diagrams: planets with the same mass and hydrogen envelope can have radii differing by 20% solely due to the oxygen-to-silicon ratio in the magma. The model also predicts that JWST can distinguish oxygen-rich from oxygen-poor interiors by the presence of water, methane, silane, and carbon dioxide lines. Compared to neighboring studies that either ignored condensation or limited themselves to one or two condensates, this work for the first time accounts for the full set of magnesium-silicate phases and their feedback on the thermal structure, leading to qualitatively new compositional bifurcations.
Next steps include accounting for non-ideality of gases and melts, which could change the solubility of hydrogen and oxygen in magma, as well as adding nucleation kinetics to test which condensates actually form. It is important to extend the model to time evolution, linking radii to planet ages and atmospheric loss processes. Laboratory experiments to determine Gibbs energies of silicate condensates at pressures up to 100000 bar and temperatures above 4000 K are critically needed to refine predictions. It is also promising to include a more complete set of elements, such as iron and sulfur, to approach the real composition of rocky exoplanets.
The results will affect the interpretation of JWST data for sub-Neptunes, models of planet formation and evolution, and understanding of the link between host-star composition and planet properties.
Conduct laboratory measurements of thermodynamic properties of silicate condensates at high pressures and temperatures. Develop evolutionary models that include condensation and atmospheric loss to compare predictions with the observed sub-Neptune population.
For planetary science, a key unsolved problem is how interior composition determines the observable atmospheric properties of exoplanets. This work shows that silicate condensation serves as a bridge between the magma ocean and the upper atmosphere, but uncertainties remain in the thermodynamics of condensates under extreme conditions, limiting prediction accuracy. Furthermore, it is unclear how quickly chemical equilibrium is established and how mixing processes in the envelope affect the delivery of condensates to observable layers.
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What the work does
This study presents the Rocky Raccoon model, which links chemical equilibrium with the thermal structure of sub-Neptunes, including the condensation of Mg, Si, O, C, and H. It shows that the composition of the magma ocean determines the mean molecular weight of the upper atmosphere and the planet's radius, and predicts the possibility of distinguishing interiors from JWST spectra.
Where its strength is
The strength of the work lies in its self-consistent treatment of the feedback between chemistry and structure: condensation alters the molecular weight gradient and the thermal profile, leading to non-convective layers and a more compact planet. This opens a path to interpreting the observed scatter in sub-Neptune radii.
Where it could go next
Worth trying
Why this may matter
Such an extension could link interior chemistry to observed atmospheric spectra and population statistics, helping to understand the formation and evolution of the most common class of planets.
Nearby works in our archive
The closest work in the archive sits at 0.64 by meaning — the usual level for our corpus.
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.