Study finds giant impacts help young rocky planets keep atmospheres

New research published in Nature Astronomy on Oct. 29, 2026 by Darius Modirrousta-Galian and Jun Korenaga, called Retention of primordial atmospheres through giant impacts, finds that giant impacts can put enough mass onto a young rocky planet to tighten its gravitational grip on its atmosphere, helping it hold on to its air.

Earlier, planetary scientists assumed the enormous heat from giant impacts would strip primordial atmospheres of hydrogen and helium from young planets as formation was ending. They wondered how planets could hold on to their atmospheres at that point. Researchers have long questioned whether young worlds such as Earth could hold on to their atmospheres during the late stages of accretion.

Earth took shape about 4.5 billion years ago as rocky planetesimals collided and merged in the young, chaotic Solar System. Impacts like these happened often late in Earth’s formation, after the disk of gas and dust around a young star had mostly disappeared and the buildup of the atmosphere had finished. This research looks at that same end stage of rocky planet formation, when big planetesimals hit growing worlds and release enormous amounts of energy.

This work indicates giant impacts could actually help young rocky worlds hold on to their atmospheres, going against the older view that such collisions strip away a planet’s air. Korenaga said giant impacts sit at the center of how rocky planets grow, because these collisions bring added mass and release heat that can drive atmospheric escape.

A giant impact has two parts: heat that can drive atmospheric escape, plus the mass the planet gains. Korenaga said rocky planets the size of Earth or larger build up through collisions between smaller planetary bodies, and whether they keep their atmospheres depends on how the added mass and the released heat combine.

When a planetesimal hits a rocky planet, part of the impact heat travels deep inside the planet, where it cannot remove the atmosphere, while part becomes mantle heat that helps strip the primordial atmosphere away. Heat carried deep into the planet does not drive atmospheric loss; only heat in the mantle helps strip the primordial atmosphere away.

Accretion of a planetesimal divides none of the gained mass; all of it strengthens the planet’s ability to keep its atmosphere. The researchers say all the accreted mass raises gravitational binding energy, but only part of the impact energy heats the mantle. Because of that imbalance, the researchers say impacts push planets toward keeping their atmospheres rather than losing them.

When mantle rock melts, it consumes energy as latent heat, the energy absorbed as rock melts. An impactor’s metallic core transfers little energy into the mantle while remaining superheated at depth. These features of how impact energy partitions explain why gravitational binding energy grows more than mantle thermal energy during a giant impact.

Giant impacts add more to a planet’s gravitational binding energy, the energy that holds a planet’s atmosphere in place, than to the heating of its mantle, moving the balance in favor of keeping the atmosphere. Bigger planets that hold on to their atmospheres come about more often through collisional growth, while smaller worlds that were not struck tend to lose their primordial atmospheres under steady radiation from their star. The results show that, across the exoplanet population, sub-Neptunes generally have interiors with more rocky mass than super-Earths do.

The results push back against the older view that giant impacts doomed planetary atmospheres and the prospects for habitability. Super-Earths and sub-Neptunes are the exoplanets astronomers detect most often, and neither type has a counterpart in our Solar System. Astronomers think super-Earths lost their atmospheres, while sub-Neptunes kept more of their primordial atmospheres.

Both kinds of planets begin as rocky cores but finish with different atmospheres. A sub-Neptune can hold on to a small fraction of its hydrogen and helium atmosphere and still grow an extended, puffy atmosphere that makes the planet much larger. Even a small retained fraction inflates the atmosphere enough to make the planet far bigger than a bare rock.

The study proposes that sub-Neptunes grew through bigger impacts that added mass and allowed them to keep thicker atmospheres, while super-Earths missed those impacts and lost their air to radiation from the host star. Modirrousta-Galian said nature produces both super-Earths and sub-Neptunes in large numbers, a puzzle the researchers called fundamental to exoplanetary science and one they set out to explain.

Jun Korenaga said the results reject the old view that giant impacts almost always strip away a planet’s entire atmosphere, showing instead that collisions like these can play a crucial role in forming sub-Neptunes. Jun Korenaga put it this way: “This overturns the decades-old canonical view that giant impacts almost always lead to complete atmospheric loss”. Their earlier work on how planetary interiors and atmospheres evolve together laid the foundation for this modeling.

Jun Korenaga is a professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences. Darius Modirrousta-Galian, who did postdoctoral work at Yale, is now based at Shanghai Jiao Tong University. Both researchers study how planetary interiors and atmospheres evolve together. The two have spent years studying how the inside of a planet and its atmosphere change together, and their earlier papers provided the groundwork for this research.

Primordial hydrogen and helium atmospheres did not themselves create habitability. Research cited by the authors indicates that these atmospheres are key to building a world that could eventually become habitable, through interactions between the atmosphere and the magma ocean, a layer of molten rock on a young rocky planet. Those interactions between magma ocean and atmosphere may have provided much of Earth’s water, and similar interactions could take place on other rocky worlds.