The Hydrogen Secret of Young Sub-Neptunes ⚡ экспресс
We present the first model of sub-Neptune interior evolution that includes miscibility of silicate melt and hydrogen. Using the H₂–MgSiO₃ phase diagram, we jointly calculate physical and chemical parameters to determine the radius of the fully mixed region. Above it lies an envelope where hydrogen and silicates refuse to mix. The binodal surface — a phase boundary — separates 'interior' and 'envelope'. Young planets can hold up to tens of percent hydrogen within, reducing their density. As they cool (from ~4000 K to ~3000 K at the binodal), both the radius and the binodal surface shrink, squeezing hydrogen into the envelope—this slows gravitational contraction. In the early stages (~10–100 Myr), silicate vapor in the envelope raises the mean molecular weight, possibly suppressing convection. After ~1 Gyr, the radii of mixed and unmixed models converge, but their internal structures remain different.
Previous models depicted sub-Neptunes as a rocky ball wrapped in hydrogen. But new research has shown: in the depths of young sub-Neptunes, due to colossal pressure and temperatures above 4000°C, hydrogen and molten rocky material mix into a single liquid, like cocoa in hot milk. Up to a third of the planet’s mass may be hidden in this ‘cocktail’.
As it cools, the mixture separates: hydrogen slowly ‘sweats out’ into the atmosphere, like bubbles in a cooling drink. This gas release slows down the planet’s contraction, making young sub-Neptunes look suspiciously puffy. Telescopes can catch this process through atmospheric features.
After a billion years, almost all the hydrogen will evaporate, and the planet will resemble an adult counterpart. But its interior forever holds traces of that hot youth — like a hidden snapshot beneath the crust.
🎯 Without this hydrogen cushion, young sub-Neptunes would look almost twice as small — their puffiness gives away a turbulent past.