Sub-Neptunes, planets sized between Earth and Neptune, are the most common in the Galaxy. But you cannot stand on them: they have no solid surface, only a dense hydrogen atmosphere. Beneath it, an ocean of molten silicate likely sloshes, the same rock that makes up Earth's mantle. Previously, it was thought that the atmosphere and ocean do not mix, like oil and water. But at pressures of millions of atmospheres and temperatures of thousands of degrees, the boundary disappears. Hydrogen dissolves into the silicate, resulting in a homogeneous supercritical fluid: neither liquid nor gas, but something in between that flows like a liquid yet fills its volume like a gas.

To understand the properties of this mixture, scientists simulated on a computer a cell of 243 atoms of magnesium, silicon, oxygen, and hydrogen. They melted the silicate, added hydrogen, and "compressed" it to pressures six million times greater than atmospheric. It turned out that dissolved hydrogen makes the mixture less dense than pure silicate. Moreover, this is not just dilution: hydrogen atoms embed themselves into the silicate structure, and the mixture behaves differently than one might expect from simply mixing two substances. At high pressure, hydrogen no longer exists as H2 molecules and integrates into the silicate network.

This discovery is important for understanding how sub-Neptunes are structured. If hydrogen distributes differently between the atmosphere and the interior, the planet's radius can differ by 5.7%. For a planet with a radius of 20,000 km, that is more than 1000 km. Now scientists have an equation of state that can be plugged into models of planetary internal structure to more accurately determine their composition from observed mass and radius. Hydrogen turns out to be not a passive impurity, but an active ingredient that changes the whole recipe.