A sub-Neptune is a planet that falls between Earth and Neptune in size. These worlds are abundant in our galaxy, but their internal structure has been hotly debated. New research shows that in their youth, these planets behave like a sponge soaked with water. Here, water's role is played by hydrogen — the lightest gas in the universe. It doesn't just sit on top like a fluffy blanket; it seeps deep into the rocky interior and mixes with molten silicates (the stuff of Earth's mantle).

This 'soaking' radically changes a planet's fate. Young stars are very active: their radiation literally blows atmospheres off nearby planets. But if hydrogen is hidden inside, the star can't get to it right away. As the upper layers escape into space, new batches of hydrogen are released from the scorching depths, like a sponge giving up liquid when squeezed. The atmosphere is replenished, and the planet stays inflated longer. Normally, after losing its envelope, a planet starts to contract rapidly under its own weight. Here, that process is delayed, as if a balloon had a hidden reserve of helium that keeps it from wrinkling.

This discovery helps explain the mysterious 'Neptune desert'—the scarcity of Neptune-sized planets on very tight orbits. Where stellar radiation is especially harsh, even the internal hydrogen reservoir eventually runs dry, and the planet loses almost its entire envelope, turning into a bare rocky core. To understand how common such hydrogen-soaked interiors are, astronomers need to measure the radii and ages of about 70–100 young planets. Their slow contraction would be a telltale sign of an internal reservoir.