Giant Impacts May Help Young Planets Keep Their First Atmospheres

A model published Oct. 26, 2026, in Nature Astronomy finds that a giant collision late in a rocky planet's formation leaves the planet more likely to keep its original hydrogen-rich atmosphere, not to lose it.

Darius Modirrousta-Galian of Shanghai Jiao Tong University and Jun Korenaga of Yale University write that the direction of that effect had been unresolved, because a giant impact does two opposing things at once: it heats the planet's interior, which drives gas off, and it adds mass, which holds the atmosphere down more tightly. Their calculation comes out on the side of the second effect.

In the model, the entire accreted mass of the impactor counts toward the gravitational hold on the atmosphere, while only part of the impact energy ends up as mantle heat. Melting rock consumes energy as latent heat, and the impactor's metal core gives up little of its energy to the mantle, instead staying, in the authors' words, "superheated at depth." Their summary of the result: "Giant impacts therefore increase gravitational binding energy more than mantle thermal energy, shifting planets towards atmospheric retention."

The authors write that larger planets with atmospheres emerge mostly through this kind of collisional growth, while smaller planets that avoid such an impact tend to lose their early atmospheres to steady radiation from their star. For the two size groups small exoplanets fall into, the paper ends on a prediction: "At the population level, these results suggest that sub-Neptunes have, on average, more massive rocky interiors than super-Earths."

The code that produces the results and the synthetic data in the paper are available via Zenodo, according to the paper's data and code availability statements.