A planet the size of Neptune, but glowing white-hot. Beneath the clouds lies an ocean of magma capable of absorbing an entire hydrogen atmosphere. The new SEAMIST model is the first to trace how such dissolution changes a planet's fate from birth to old age.
The big surprise is the mechanism of catastrophic boiling. When magma releases its stored hydrogen back into the atmosphere, the atmosphere puffs up and begins to escape rapidly under the star's radiation. A feedback loop emerges: the more gas escapes, the stronger the heating and mass loss. This process can kick in even billions of years after the planet formed.
The picture is different with complete mixing. If hydrogen dissolves freely in magma, the planet holds onto its gas envelope much longer. This is especially noticeable around low-mass stars: there should be many large planets with thick atmospheres. But telescopes don't see them yet. Partial dissolution fits the observed data better and explains the sharp drop in the number of planets with radii around four Earth radii.
The scorching planet resembles a forge: metals and rock melt, while hydrogen is either absorbed into the melt or bursts out. Whether the planet remains a gas giant or turns into a rocky ball depends on how deeply hydrogen penetrates the magma.
The magma ocean on a sub-Neptune can reach depths of thousands of kilometers — hundreds of times deeper than all of Earth's water.