When Impacts Bring Back the Air: How Collisions Could Revive Atmospheres on M-Dwarf Worlds
Recent studies have revealed that many small, rocky planets orbiting M-dwarf stars, cool, dim stars that make up most of the stars in our galaxy, might not have stable atmospheres. Prune C. August and collaborators explore a fascinating possibility that even after an atmosphere collapses, meteorite impacts could periodically bring it back. Their work uses models to show how such impacts can temporarily recreate thick layers of gas, meaning a planet’s atmosphere could appear and disappear over time rather than evolve smoothly.
Atmospheric Collapse on M-Dwarf Planets
Planets orbiting close to M dwarfs often become tidally locked, with one side permanently facing the star while the other stays in darkness. August explains that this imbalance can cause gases like carbon dioxide (CO₂) to condense and freeze on the frigid nightside. As the atmosphere thins from stellar radiation and cooling, a feedback loop begins: the colder it gets, the more CO₂ turns to ice, leading to a total “atmospheric collapse.” This process has been theorized for decades and may explain conditions on worlds like Mars or even Mercury’s volatile-rich surface. The frozen gases, though inactive, form vast reservoirs that could, under the right conditions, be revived.
How Impacts Reinflate Atmospheres
The authors describe a sequence where a meteorite impact provides the necessary energy to vaporize the stored ice and rock on the planet’s surface. The resulting hot vapor plume can melt additional ice, rain molten silicates, and temporarily warm the entire planet. August’s team calculates that impacts from objects roughly 5–10 kilometers wide could be sufficient to “reinflate” an atmosphere made mostly of CO₂. These events would release the trapped volatiles, creating a transient atmosphere that might persist for millions of years before collapsing again. Impacts that are too small or too infrequent would not trigger regeneration, while too many would prevent ice buildup.
Modeling the Rhythms of Collapse and Renewal
To explore this process, the researchers modeled each planet as a two-state system: “inflated,” when it has an atmosphere, and “collapsed,” when the atmosphere has frozen out. They incorporated volcanic outgassing (which slowly adds gas) and atmospheric escape caused by stellar radiation (which removes it). Impacts, occurring randomly according to statistical distributions, act as reset buttons, restarting an atmosphere when conditions are right. Using parameters based on Earth’s volcanic CO₂ emissions and typical impact frequencies, August ran simulations covering billions of years of planetary history.
Applying the Model to Real Worlds
The study focuses on three planets targeted by the James Webb Space Telescope’s Rocky Worlds program: GJ 3929 b, LTT 1445 Ab, and LTT 1445 Ac. Each planet orbits close to its small red star and may have lost any original hydrogen–helium atmosphere long ago. The team’s results show that under certain conditions, these worlds could spend a large fraction of their lifetimes with transient CO₂ atmospheres regenerated by impacts, up to 70% for GJ 3929 b and 80% for LTT 1445 Ab. The balance depends on the planet’s distance from its star, the intensity of radiation, and the rate of impacts over billions of years.
Why Collapsed Atmospheres Might Be a Good Thing
Surprisingly, August and her colleagues found that atmospheric collapse can help preserve volatiles. When gases freeze out, they stop escaping into space, protecting material that could later re-form an atmosphere. Their analysis also shows that moderate impact frequencies (roughly one to one hundred per billion years) are most effective for sustaining transient atmospheres. Too few impacts lead to permanent collapse, while too many prevent the ice from ever building up. The study also notes that similar processes might apply to atmospheres made of other gases, like nitrogen or water vapor, though these would require more complex modeling.
Implications for Observations
This work reshapes how scientists might interpret observations of rocky exoplanets. Instead of assuming that an airless planet has permanently lost its atmosphere, it may simply be in a collapsed phase between impact events. Because these atmospheres can appear and vanish on geological timescales, detecting one depends on catching the planet at the right moment. The authors suggest that future telescope surveys, like those conducted by JWST, could use the probability of transient atmospheres as a guide for selecting targets. For instance, a planet that spends even 10% of its lifetime with an atmosphere still has a fair chance of being observed during an inflated phase.
Conclusion
August and collaborators conclude that the evolution of terrestrial planet atmospheres is not necessarily a smooth decline from thick to thin, but an episodic cycle of collapse and rebirth. Meteorite impacts can temporarily restore lost atmospheres, particularly on planets around M dwarfs where ice reservoirs accumulate on the nightside. This discovery means that the absence of an atmosphere today does not rule out its presence in the past, or its return in the future. For astronomers, that insight offers a new lens through which to interpret the surprising diversity of worlds now being revealed across our galaxy.
Source: August
