Artists' impressions of a super-Earth (Kepler-452b) and a sub-Neptune (Kepler-22b). Credit: NASA

A new study lets some of the air out of a long-held theory that growing, rocky planets will lose their atmosphere if they smash into a large object in space.

In fact, these kinds of collisions often help protoplanets hold on to their hydrogen-rich primordial atmosphere, say researchers Jun Korenaga of Yale and Darius Modirrousta-Galian, a former Yale postdoc who is now at Shanghai Jiao Tong University.

"Giant impacts are a key part of how rocky planets grow," said Korenaga, a professor of Earth and planetary sciences in Yale's Faculty of Arts and Sciences and co-author of the new study in the journal Nature Astronomy .

"Planets the size of Earth or larger are expected to form through collisions between smaller planetary bodies," Korenaga said. "These collisions add mass and release enormous amounts of heat, which can drive atmospheric escape.…