Webb found evidence of air on a lava world, the coldest yet known to show any

A rocky planet that circles its star once a day, 154 light-years from Earth, appears to be holding on to an atmosphere. University of Chicago astronomers report that the James Webb Space Telescope found evidence of air on HD 3167 b, which they describe as the coldest lava world found so far with signs of one.

The wording matters. The team calls it evidence, not a confirmed detection: no molecule has been identified, and the release gives no statistical significance. What Webb measured was heat, and the case for an atmosphere rests on how that heat falls short of what a bare rock would give off. That is an indirect route to air, and the authors present it as such rather than as a spectrum showing a particular gas.

How a cool dayside points to an atmosphere

HD 3167 b is a rocky super-Earth in the constellation Pisces, and its orbit is so tight that its year lasts about one Earth day. Planets in that position are baked into lava worlds, with surfaces hot enough to melt rock, as laid out in the ScienceDaily release on the study. The usual expectation is that such a world has had its air stripped away by its star.

Webb watched the planet slip behind its host star, a K-type star according to the university’s image caption, in a technique called a secondary eclipse. By measuring how much mid-infrared light vanished during that disappearance, the team could work out how much the planet itself was emitting, and so how hot its daytime face really is. The release does not name the Webb instrument used or the number of eclipses observed.

The result was cooler than theory allows for a bare, airless rock at that distance. The article quotes no temperature, only that the dayside sits noticeably below its theoretical maximum. An atmosphere can explain the shortfall in two ways: winds could carry heat around to the night side, much as Venus spreads its heat, or the air could host clouds that bounce starlight back into space.

The narrow superlative: coldest lava world with evidence of air

The superlative is narrow. According to the University of Chicago article, HD 3167 b is the coldest lava world found so far with evidence of an atmosphere. It is not the coldest rocky planet to hold air, and it is not a claim about rocky planets generally. As of the October 2026 release, the other lava worlds with atmosphere evidence are all ultra-hot, which is what makes this one a first of its kind rather than a new rule.

Brandon Park Coy, the graduate student who led the work, told the university that if a planet has an atmosphere, it might also have clouds that reflect incoming starlight and cool the dayside. He added that the makeup of HD 3167 b’s air is still up in the air. Edwin Kite, a UChicago associate professor of geophysical sciences and a co-author, said of such worlds, “These planets are too hot for life.”

Kite also framed the surprise plainly: “What’s so surprising is that the closer a rocky planet orbits its star, the harder it should be to have an atmosphere”. That tension is the reason a cooler-than-expected dayside draws attention, because the planet sits in the part of the map where air should be hardest to keep.

Webb’s ten-world lava survey and the hunt for a temperature threshold

The result belongs to a wider program. Megan Weiner Mansfield, who earned her doctorate at the university in 2021 and now works at the University of Maryland, leads a Webb survey of ten ultra-hot lava worlds. Its aim is to learn whether a critical temperature separates planets that keep an atmosphere from those that do not. A cooler planet with apparent air moves that line, though one world cannot draw it alone.

Coy and his co-authors, among them Qiao Xue, Jacob Bean, Eliza Kempton and Kite, published the work as “Evidence for an Atmosphere on the Ultra-short-period Super-Earth HD 3167 b” in The Astrophysical Journal Letters (volume 1005, issue 2, L77), with the paper at its DOI and on the journal’s site. The summaries available do not carry a named outside critic, so the main caution comes from the authors, who say more observations are needed to resolve the composition.

The stake reaches beyond one hot rock. Coy said early Earth, in its magma-ocean stage, may have looked a lot like a lava world, so studying how such planets hold or lose air could inform how rocky planets get their atmospheres at all. Kite added that these worlds are too hot for life but can reveal the processes that matter. Telescope details for the observatory are on NASA’s Webb mission page.

Until the composition is pinned down, HD 3167 b remains an inference built on missing heat. Follow-up spectra that name a gas such as carbon dioxide, carbon monoxide or water, the heavier candidates the team raises for lava worlds, would turn that inference into a firm detection. Until then, the careful reading is the one the authors themselves use: strong evidence, an open question about what the air is made of, and a new data point for the temperature threshold that the wider survey is trying to locate.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview