Coldest Lava Exoplanet Defies Theory by Retaining Atmosphere

For decades, astronomers have watched lava worlds—planets orbiting so close to their stars that their surfaces melt into molten rock—and noticed a pattern that seemed ironclad: the hotter ones kept atmospheres, the cooler ones did not. It made intuitive sense. Extreme heat vaporizes rock and surface material, creating a gaseous envelope around the planet. Remove enough of that heat, and the atmosphere should vanish. But HD 3167 b, an Earth-sized exoplanet 154 light-years away, appears to break that rule.

Using NASA's James Webb Space Telescope, an international team of researchers observed HD 3167 b during a secondary eclipse—the moment when the planet passes behind its star—and found something unexpected. The exoplanet's surface temperature was significantly cooler than models had predicted, yet the data suggested it might still retain an atmosphere. The discovery, published in The Astrophysical Journal Letters, challenges what scientists thought they understood about how these extreme worlds hold onto their gases.

HD 3167 b is no ordinary exoplanet. It completes an orbit around its K-type star—a star smaller and cooler than our Sun—in just 0.96 Earth days, meaning it whips around its host star faster than Mercury orbits ours. The planet itself is roughly 1.6 times Earth's radius and about 4.8 times Earth's mass. Because it orbits so close to its star, it is tidally locked, with one hemisphere perpetually facing the star and the other locked in permanent darkness. The dayside bakes under relentless radiation. The nightside freezes in shadow. Yet the researchers' observations suggest heat from the illuminated side may be redistributing across the planet to the dark side, a process that could fundamentally alter how these worlds lose or retain their atmospheres.

Brandon Park Coy, a PhD student in geophysical sciences at the University of Chicago and lead author of the study, frames the significance differently. "Despite how inhospitable they are for life, we're also interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world," he explained. In the solar system's infancy, when terrestrial planets were still forming, Earth itself went through a magma ocean stage—a period when the entire surface was liquid, heated by the energy of countless planetesimal collisions. Lava worlds offer a window into those conditions, a chance to study what our own planet endured in its first few million years of existence.

The distinction between lava exoplanets and the solar system's own volcanic world matters. Jupiter's moon Io, often cited as a terrestrial analog, owes its volcanic fury to tidal heating—the gravitational squeeze from Jupiter and the other Galilean moons. Lava exoplanets operate on a different principle entirely. Their surfaces melt because they orbit so close to their stars that the stellar radiation itself liquefies rock and stone. The vaporized material rises and forms an atmosphere. Or at least, that is what the models said should happen.

HD 3167 b complicates that picture. If a cooler lava world can maintain an atmosphere through heat redistribution rather than surface vaporization alone, then the mechanisms governing atmospheric retention on these planets are more nuanced than previously understood. The finding opens new questions about planetary formation and evolution across the exoplanet population. It suggests that temperature and composition alone may not determine whether a lava world keeps its atmosphere—that the way a planet moves heat around its surface, and how that heat escapes to space, matters just as much. As researchers continue to observe more of these worlds with increasingly sophisticated instruments, the rules governing lava planets will likely continue to shift.