A planet without an atmosphere... until something hits it!

The atmosphere; that protective blanket of gases and aerosols that shields us from cosmic radiation, burning meteors, and makes life on Earth possible. We often take it for granted, but most rocky exoplanets (planets with a solid surface like Earth) likely lost their atmospheres long ago, early in their formation histories… or did they? New research suggests that the atmospheres of rocky exoplanets might be far more dynamic than we imagined, periodically depleting and then rebuilding over time. Join us in today’s astrobito to learn more about this intricate process!

Paper details:

Title: Atmospheric collapse and re-inflation through impacts for terrestrial planets around M dwarfs

Authors: Prune C. August, Robin Wordsworth, Mikayla Huffman, et. al.

First-author institution: Technical University of Denmark, Department of Space Research and Technology

Status: Open access arXiv.

For years, the astronomy community assumed that small rocky worlds orbiting the Galaxy’s most common stars, M dwarfs, were doomed to be barren. Bathed in harsh stellar radiation and tidally locked so that one side faces eternal day and the other endless night, these planets should lose their atmospheres early and never get them back.

But new research suggests that this bleak picture might be missing something dramatic: meteorite impacts could periodically reinflate their atmospheres, giving them fleeting breaths of air that last millions, or even billions, of years.

Lessons from familiar worlds

We already know that meteor impacts shape planets. Earth’s Chicxulub impact (the infamous event that vanished dinosaurs from our world) vaporised rock, triggered global climate change, and reset ecosystems. On Mars, on the other hand, researchers have proposed that basin-scale collisions could explain ancient valley networks by injecting heat and volatiles into a once-thin atmosphere.

But not only meteor collisions have a say on the state of an atmosphere. In our Solar System, we see bodies whose atmospheres collapse and re-inflate as conditions change. Io’s sulfur dioxide (SO2) atmosphere literally disappears when the moon slips into Jupiter’s shadow and reappears moments later!

These are some examples that show us how having an atmosphere is not necessarily a constant throughout a planet’s lifetime.

Trapped atmospheres that only an impact can set free

Picture this: a rocky planet parked close to an M dwarf. Daily temperatures on the sunside soar, while the nightside, locked forever away from starlight, plunges into deep cold. Over time, high-energy radiation strips the atmosphere away. Once the pressure drops below a critical point, the remaining gases condense and freeze out on the frigid nightside, forming vast ice sheets rich in atmospheric gases.

The result is a collapsed atmosphere: an atmosphere that freezes and deposits itself onto the surface of the planet or moon it belongs to. To telescopes like JWST, the planet now looks airless with a non-existing atmosphere. But all those volatiles are still there, trapped as ice on the dark surface.

A few hundred million years pass. Then a comet or 5-10-kilometre asteroid collides onto the nightside of the planet.The collision is brief, but the energy released is enormous. Ice vaporises instantly. A glowing plume of rock and gas radiates heat across the planet’s surface, triggering runaway melting and sublimation. What follows is a short-lived hydrologic cycle: clouds form, acidic rains fall, water re-evaporates off a planet-wide sheet of molten rock. And suddenly, the planet has an atmosphere again: thick enough to warm the nightside, stable enough to persist!

Schematic representation of the suggested dynamic life-cycle of a rocky planet’s atmosphere.

This is the scenario that the group behind this research is suggesting. One where the atmosphere of tidally-locked planets around M dwarfs gets dynamically depleted and re-established as the planet gets hit by meteorites over its lifetime.

Simulating a dynamic atmosphere

To understand how often these reborn atmospheres might exist, the authors ran tens of thousands of simulations of planets orbiting M dwarfs at different distances, under different bombardment and volcanic outgassing rates. Instead of asking whether a planet has an atmosphere today, they asked a smarter question: How often does this planet have an atmosphere over its entire lifetime?

The results are surprisingly optimistic. At moderate impact rates (around 1-100 collisions per billion years), the mechanism thrives. Too few impacts, and the ice reservoir never gets tapped. Too many, and the planet never has time to stockpile volatiles. But at just the right tempo, impacts regenerate thick CO2 atmospheres that can linger for tens to hundreds of millions of years at a time. In some cases, especially further from the star, these transient atmospheres can persist for over a billion years.

The simulations show that when the planet is not too close to its host star and the impactor diameter is big enough (bottom two panels), it triggers the re-vaporisation of the atmosphere, which in some cases can last for billions of years. Figure adapacted from Figure 3 in the main paper.

Counterintuitively, and interestingly enough, collapsing an atmosphere may help save it. Once gases freeze out on the nightside, they are shielded from the very stellar radiation that would otherwise strip them away. Instead of escaping to space, they hibernate. And when the moment comes, via volcanism, outgassing, or an unlucky comet, they return.

A new way to think about rocky worlds

Traditionally, we’ve pictured atmospheric evolution as a smooth curve: thick at birth, thinning over time, ending either with a breathable blanket or with nothing at all. This work suggests something messier and far more dynamic. Atmospheres might flicker on and off over billions of years, governed by stochastic impacts, changing stellar outputs, and the slow drip of internal geologic activity.

For the astronomy community, this has major implications:

A non-detection of an atmosphere today doesn’t guarantee a barren world.
Worlds across the universe could be caught in different phases of their atmospheric cycles.

And as JWST ramps up its 500-hour “Rocky Worlds” programme, these ideas are timely.

What’s next?

These results could change the way we study rocky planets. Future missions might focus on searching for chemical traces of past atmospheres on these worlds: oxidation patterns on their day sides, spectral hints of recondensed volatiles, and subtle signs of recent atmospheric collapses. Likewise, it will be essential to determine the impact rates in these exoplanetary systems, as well as to model atmospheres that contain not only carbon dioxide (CO2), but also water, nitrogen, and oxygen.

If this hypothesis is correct, then right now, somewhere in the universe, a rocky exoplanet orbiting an M-dwarf star might be awaiting its next impact to debut a brand new, fresh atmosphere.

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