A young sub-Neptune is a pressure cooker where hydrogen and rock simmer in the same pot. Six Earth masses, three percent hydrogen by mass, and interior temperatures above 4000 K. At that heat, silicate melt and hydrogen don't just coexist: they mix completely, forming a single supercritical fluid. There is no boundary between core and envelope. There is only the binodal, an invisible surface on the phase diagram where the mixture decides whether to stay homogeneous or separate into gas and rock.

Previously, it was thought that a sub-Neptune was structured like a layered cake: a dense silicate core, a hydrogen-helium envelope on top, and at the boundary gases only slightly dissolve in magma according to Henry's law. But first-principles calculations showed that at temperatures above roughly 4000 K, solubility is no longer "slight." Hydrogen and molten silicate mix in any proportion. The authors of the new work built a model in which the planet itself determines where the boundary lies, following the binodal of the H2–MgSiO3 phase diagram. While the planet is hot, the binodal lies deep, and almost all hydrogen is hidden in the interior. As it cools, the binodal rises, and hydrogen gradually escapes from the silicate part, like steam from cooling porridge.

This simple picture changes everything. In the standard model, the planet loses energy mainly through cooling of the hydrogen envelope. Here, the main source is gravitational compression of the silicate part. Hydrogen mixed with rock makes the interior less dense, so the silicate material sits higher in the gravitational potential. When the planet cools and hydrogen leaves the interior, the silicate core contracts, releasing heat. Therefore, a miscible planet cools more slowly. At ages of 10–100 million years, its radius is tens of percent smaller than that of an immiscible model with the same parameters. After 5 billion years, the radii nearly match, but the interior is arranged differently: in the miscible model, hydrogen is distributed throughout the planet, not concentrated in the envelope.

This work introduces the binodal as a physically justified boundary between the interior and envelope of a sub-Neptune, instead of an arbitrary magma ocean surface. This changes the interpretation of observed radii: the hydrogen mass in the planet may be correct, but the envelope mass may be overestimated by several times, because part of the hydrogen is hidden in the interior. Extrapolating Henry's law to high temperatures misses the phase transition and leads to errors in estimating the interior composition. The model can be used to interpret the radii of young sub-Neptunes and refine estimates of their envelope masses.

The next step is to include iron, helium, and water in the model, which require new calculations of phase equilibria. It is also important to link miscibility to gas accretion and atmospheric loss processes: if hydrogen goes into the interior, the "boiling" of the envelope after disk dispersal may proceed differently. Observational tests may emerge when comparing the radii of young sub-Neptunes in clusters with predictions from models with and without miscibility.