Why some lava-covered exoplanets retain atmospheres for billions of years

An artist's rendering of "cosmic sandbar" planet TOI-561b orbiting its star, with its atmosphere illuminated. Credit: NASA, ESA, CSA, Ralf Crawford [STScI] Stanford researchers have offered an explanation for why some sizzling, lava-covered worlds nestled tightly around their stars can retain their atmospheres. The study, published in The Astrophysical Journal Letters, shows that lava worlds can maintain thick gaseous envelopes for billions of years thanks to molten rock sloshing over their surfaces. The lava slows the release of gases from the planets' interiors into the atmosphere, balancing atmospheric loss caused by stellar radiation. The findings extend the "cosmic shoreline" framework, which models how close rocky planets can get to their stars while retaining their atmospheres. The researchers call this region the "airless valley."

• The study identifies the "cosmic sandbar" as a new regime beyond the "cosmic shoreline," where outgassing balances atmospheric loss.
• The model incorporates gas exchange between an atmosphere and a molten lava surface, considering escape of gas into space and cooling and solidification of the lava.
• Planets in this regime, like 55 Cancri e, are generally hefty super-Earths with surfaces covered by lava.

The research challenges the traditional cosmic shoreline concept by showing that some lava worlds sit far closer to their stars than the cosmic shoreline and still have atmospheres. The study suggests that the "cosmic sandbar" is a new boundary where outgassing balances atmospheric loss.

• The researchers propose that planets between the shoreline and the sandbar sit close enough to their star for their atmospheres to be stripped away but cool off too quickly to replenish them.
• The study expands our understanding of the boundary between airless worlds and those capable of sustaining an atmosphere.
• The new model provides a framework for determining where the cosmic shoreline lies for specific stars and planets.

The findings have implications for understanding planetary habitability and the factors that determine whether a planet can retain its atmosphere despite intense stellar radiation. The study highlights the importance of considering outgassing as a mechanism for maintaining atmospheres on close-in rocky planets.

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