This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Stanford researchers have offered an explanation for why some sizzling, lava-covered worlds nestled tightly around their stars can retain their atmospheres. At such close quarters, intense stellar radiation should strip away a planet's air.
Based on modeling of planetary atmospheres and interiors, a new 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.
As Earth's shorelines represent the boundary between land and water, the cosmic shoreline represents the boundary between having an atmosphere and not having one. However, some lava worlds sit far closer to their stars than the cosmic shoreline and still have atmospheres, challenging the framework. "These lava worlds have pointed to something being wrong with the cosmic shoreline boundary, but we've found a way for them to preserve their atmospheres by proposing a new regime beyond it," said lead study author Barron Nguyen, a graduate student in the lab of Laura Schaefer at the Stanford Doerr School of Sustainability.
The name proposed by Stanford researchers and their collaborators for this new realm is the "cosmic sandbar," analogous to the sandy ridges that form offshore in Earthly oceans. 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 after formation to replenish them. The researchers call this region the "airless valley." "Where the shoreline boundary is between airless worlds and those capable of sustaining an atmosphere has been a major open question in planetary science," said Schaefer, the study's senior author and an assistant professor of Earth and planetary sciences in the Doerr School of Sustainability.
"The new model expands our understanding of this boundary and the factors that go into determining where it lies for specific stars and planets." In our solar system, the strategy in the search for extraterrestrial life has long been to follow the water. But for worlds beyond our solar system, a different strategy is required: follow the atmospheres. "Scientists have been interested in figuring out which planets have atmospheres and which do not, because that's the first step of looking at planetary habitability," said Nguyen.
The cosmic shoreline concept emerged over the past decade as a promising method of identifying potentially hospitable worlds for follow-up observation. However, scientists have recently begun calling it into question because of planets like 55 Cancri e, a "super-Earth" nearly eight times Earth's mass that orbits about 20 times closer to its star than Mercury does to our sun. In 2024, the James Webb Space Telescope revealed that 55 Cancri e possesses a stunningly thick atmosphere.
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