Planetary collisions may help young worlds hold on to their atmospheres

Planetary collisions may help young worlds hold on to their atmospheres Gaby Clark Scientific Editor Robert Egan Senior Editor 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. Their effect on a planet's atmosphere depends on how those two changes work together." The new research also sheds light on the predominance of two types of planets beyond Earth's solar system: "super-Earths," exoplanets that are slightly larger than Earth and have no primordial atmosphere, and "sub-Neptunes," which are even larger and do have a primordial atmosphere. "Why nature produces both types in such abundance is a fundamental puzzle in exoplanetary science," said Modirrousta-Galian, a former postdoctoral researcher in Korenaga's lab and lead author of the study. "We set out to find an explanation." Added mass strengthens gravity's hold Over the past few years, Modirrousta-Galian and Korenaga have written a series of papers laying out the foundation for how atmospheric escape and planetary interiors should coevolve. The new study brings these studies together to conceptualize what happens after a giant collision. The researchers found that after a collision, the added mass increases a planet's gravitational binding energy—and its hold on the atmosphere. And the extra heat? Only part of the energy created by impact raises temperature in the planet's mantle (its rocky middle layer), while the metallic core of the impacting object remains superheated deeper down, they said. Collisions favor sub-Neptunes The result of all of this, say the researchers, is that large sub-Neptunes with thick atmospheres emerge preferentially via collision. Meanwhile, small super-Earths that don't encounter such giant collisions tend to lose their atmospheres after prolonged stellar irradiation. "This overturns the decades-old canonical view that giant impacts almost always lead to complete atmospheric loss, demonstrating instead that these collisions can play a crucial role in the formation of sub-Neptunes," Korenaga said. Publication details Darius Modirrousta-Galian et al., Retention of primordial atmospheres through giant impacts, Nature Astronomy (2026). DOI: 10.1038/s41550-026-03006-1 Journal information: Nature Astronomy Provided by Yale University