Webb found strong evidence of an atmosphere on a rocky world with a one-day year

HD 3167 b circles its star once every 0.96 days, a year that lasts less than an Earth day, and a measurement from the James Webb Space Telescope says the rocky planet is cooler on its sunlit side than bare rock could possibly be. The University of Chicago team behind the result calls it strong evidence that the world is wrapped in an atmosphere.

The planet sits 154 light-years away in the constellation Pisces and is classed as a lava world. According to the university’s account, it is the coldest lava world found so far with evidence of an atmosphere, and the Webb observation behind that claim was published in The Astrophysical Journal Letters in 2026.

A secondary eclipse and a 38-parts-per-million dip

The team used Webb’s mid-infrared instrument, MIRI, in its low-resolution spectroscopy mode, to watch HD 3167 b slip behind its star, an event called a secondary eclipse. When the planet vanishes, the light lost is the planet’s own mid-infrared glow, and that glow indicates how hot its dayside is. According to the paper’s arXiv listing, the white-light eclipse depth came out at 38 plus or minus 11 parts per million, more than 5 sigma below what a dark, maximally hot bare rock would produce.

The planet is a super-Earth of about 1.6 Earth radii with an equilibrium temperature of 1,786 kelvin, per the same abstract. Lead author Brandon Park Coy, a University of Chicago graduate student in Edwin Kite’s group, described the dayside as noticeably cooler than its expected maximum. The paper, “Evidence for an Atmosphere on the Ultra-short-period Super-Earth HD 3167 b,” appears in The Astrophysical Journal Letters, volume 1005, page L77.

The university’s account was republished by Astrobiology.com on Sept. 3, 2026, and later carried by ScienceDaily on Oct. 7. The preprint itself was posted in April 2026, and an Astrobiology.com summary dated April 17 already listed the same depth and orbital figures.

Two ways an atmosphere cools the dayside

A bare rock with no air would soak up starlight and radiate it straight back from its day side, reaching the theoretical maximum temperature. Space.com adds that, as is common among lava worlds, one side of the planet always faces its star, so the sunlit hemisphere that Webb measured is a permanent dayside rather than a patch that rotates in and out of heat. HD 3167 b falls short of that. The authors attribute the shortfall to an atmosphere acting in either of two ways, or both: clouds or other material reflecting starlight before it can heat the surface, or winds carrying heat around to the nightside.

The paper also notes that the planet is slightly under-dense relative to an Earth-like composition, a trait it describes as consistent with an atmosphere. It labels HD 3167 b the least irradiated ultra-short-period super-Earth with evidence for one.

The wording is deliberately graded. The paper’s title says “evidence,” and the university’s account supplies the adjective “strong.” Neither claims a detected gas, and the release states that the atmosphere’s makeup is still uncertain. The emission spectrum is too imprecise to say what the atmosphere is made of, and the authors recommend follow-up observations with Webb’s NIRSpec instrument.

Lava worlds that keep air, and the Maryland-led Webb program

Among more than 6,300 cataloged exoplanets, only a handful of rocky planets appear to hold atmospheres. The university counts five terrestrial planets with atmospheres, and the other four are in the ultra-hot regime. HD 3167 b is cooler than those four, which is why astronomers care: atmospheres had so far turned up only among the hottest rocky planets, and this one extends the list to a colder case, raising questions about how and at what temperatures such atmospheres form.

Expectations had been that any air on such a planet would be vaporized rock. The release says researchers now see signs that some lava worlds may hold heavier gases such as carbon dioxide, carbon monoxide or water, though the composition remains unresolved and the planet may have a silicate-rich makeup similar to Earth’s mantle.

Megan Weiner Mansfield, who earned her doctorate at Chicago in 2021 and is now at the University of Maryland, leads a Webb program studying 10 ultra-hot lava worlds to find the temperatures at which they tend to keep atmospheres, according to Space.com’s Sept. 29 coverage. Coy has pointed out that early Earth may have resembled a lava world during its magma-ocean stage, so a planet like this may speak to conditions in the first few million years of Earth’s history. Kite says such planets are too hot for life, but they offer a view of processes relevant to other rocky worlds, including the magma-ocean stage early Earth is thought to have passed through.

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

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