Core-envelope miscibility in sub-Neptunes and super-Earths - Nature

"The population of super-Earths and sub-Neptunes, and the origin of the radius valley that separates these two classes of planets, is best explained by cores that are made of an Earth-like composition without a substantial amount of accreted ice8,9,10,11. For sub-Neptunes, the hydrogen-rich envelope overlies the rocky core for billions of years, whereas for super-Earths, the envelope may be retained for about 100 Myr (refs. )."

"The structure and evolution of sub-Neptunes and super-Earths challenge our understanding of fundamental properties of materials at extreme conditions. For example6, at the core-envelope boundary of a typical sub-Neptune, the temperature may be 5,000 K, whereas the pressure may be 5 GPa. Both core and envelope are fluid at the conditions of the interface, which exceed the silicate melting temperature: the rocky core forms a magma ocean."

Both sub-Neptunes and super-Earths likely accreted hydrogen-rich primary envelopes from the protoplanetary disk. Sub-Neptunes retained substantial portions of these envelopes over billions of years, whereas super-Earths lost their envelopes within roughly 100 Myr. Cores composed of Earth-like, relatively ice-poor materials explain the observed radius valley separating the two populations. During envelope retention, hydrogen and molten rocky cores can undergo chemical reactions that alter atmospheric and interior compositions. Typical core-envelope boundary conditions in sub-Neptunes reach ~5,000 K and ~5 GPa, producing a magma ocean at the interface. These pressure-temperature regimes remain poorly explored experimentally and theoretically.

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