Stanford researchers have offered a compelling explanation for why some sizzling, lava-covered worlds nestled tightly around their stars can keep hold of 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 the molten rock sloshing over their surfaces. The lava slows the release of gases from the planets' insides into the atmosphere, balancing out the loss of atmosphere due to 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 atmosphere to be stripped away, but cool off too quickly after formation to replenish that atmosphere. 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."
Seeking planets on the cosmic shore
In our solar system, the strategy in the search for extraterrestrial life has long been to follow the water. But for worlds beyond, 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. And in the past few months, a flurry of new observations has showcased a growing list of similar lava worlds with thick atmospheres.
To explain these counterintuitive planets, Nguyen and colleagues assembled a model that simulates the exchange of gas between an atmosphere and a molten lava surface, while accounting for the escape of gas from the atmosphere into space and for the cooling and eventual solidification of the lava. They then incorporated the atmosphere-retaining lava worlds into the model, alongside other known exoplanets and solar system worlds for comparison.
Sustaining an atmosphere against the odds
The model indicated that atmospheres can primarily be regulated either by the escape of gas into space or by the release of gas from the planet into the air (known as "outgassing"). The cosmic sandbar encompasses those hot, close-in worlds on which outgassing balances out escape.
The planets in this regime, like 55 Cancri e, generally are hefty super-Earths with surfaces covered by lava. Planets slightly farther from their stars, like Mercury, tend to cool off and solidify more quickly, trapping their gases underground. Unable to replenish their diminishing atmospheres, these planets populate the airless valley. Only at sufficient distances from their stars – that is, on the cosmic shoreline – can smaller, cooler planets like Venus and Earth avoid having their atmospheres burnt off by stellar radiation.
The Stanford researchers look forward to seeing how future exoplanetary surveys will flesh out and build upon the cosmic shoreline framework, newly amended with the sandbar concept.
"A major takeaway from our study is that the cosmic shoreline isn't a lost cause," says Nguyen. "There had been some pessimism about it because of these lava worlds, but now we know there's a broader set of parameters that can enable a planet to generate and maintain an atmosphere."
Barron Nguyen is a 2026 U.S. National Science Foundation Graduate Research Fellow.
Co-authors Bo Peng and Michelle Hill are postdoctoral scholars and Andrea Zorzi is a graduate student in Earth and planetary sciences at the Stanford Doerr School of Sustainability. Additional co-authors are affiliated with The University of Chicago; University of California, San Diego; University of Hawai'i at Mānoa; University of California, Santa Cruz; and University of Maryland, College Park.
