Smashing into another world may be how young planets keep their skies

Collisions were supposed to strip young planets bare. A new study in Nature Astronomy from Yale's Jun Korenaga and Darius Modirrousta-Galian finds the opposite is often true: when a growing rocky planet is hit by a large object, the extra mass strengthens gravity's grip, and much of the impact's heat sinks into the newcomer's metal core instead of boiling the air away. That helps explain why the galaxy is full of sub-Neptunes — worlds bigger than Earth wrapped in thick atmospheres — while smaller super-Earths that avoided big collisions slowly lost theirs. Violence, in planet-building, turns out to be a keeper of atmospheres.