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. ).