Researchers using NASA’s James Webb Space Telescope have detected atmospheric evidence on HD 3167 b, an Earth-sized lava world orbiting its host star in less than a single Earth day. Located 154 light-years away in the constellation Pisces, the exoplanet defies standard planetary models by retaining an atmosphere despite being bombarded by stellar radiation, according to a study published in The Astrophysical Journal Letters.
Detecting an Atmosphere on an Ultra-Short-Period Super-Earth
HD 3167 b possesses a radius of about 1.6 times that of Earth and a mass roughly 4.8 times greater. It completes an orbit around its K-type host star in approximately 0.96 Earth days. Because the planet sits so close to its star, it is tidally locked, meaning one hemisphere permanently faces the heat while the other remains in perpetual darkness.
For years, astronomers categorized rocky planets orbiting this close to their stars as bare, airless rocks stripped clean by stellar winds and high-energy photons. However, observations of the planet’s secondary eclipse—captured when the world passes directly behind its star—revealed that the dayside of HD 3167 b is significantly cooler than theoretical models predicted for an airless body.
“What’s so surprising is that the closer a rocky planet orbits its star, the harder it should be to have an atmosphere, because it’s bombarded by stellar wind and gets more high-energy photons from the star. But it seems that many of these lava worlds do,” said Edwin Kite, associate professor of geophysical sciences at the University of Chicago and co-author of the study.
How Heat Redistribution Reveals Atmospheric Presence
When an exoplanet lacks an atmosphere, its star-facing side absorbs intense radiation and reaches maximum theoretical temperatures based on its surface reflectivity and distance. The presence of an atmosphere changes this dynamic by redistributing thermal energy from the day side to the night side, similar to the atmospheric circulation observed on Venus.
The research team, led by University of Chicago graduate student Brandon Park Coy, targeted HD 3167 b to investigate the temperature boundaries that separate rocky exoplanets with atmospheres from those without. The project is part of a broader observational program examining ten ultra-hot lava worlds to pinpoint a critical temperature threshold where atmospheric retention becomes possible.
“This planet, however, is the coldest lava world found so far with evidence of an atmosphere,” said Coy. “This is interesting because we’re trying to understand the temperature transition between planets with and without atmospheres.”
Windows Into Early Planetary Evolution
While extreme lava worlds are entirely uninhabitable, studying their mechanics offers planetary scientists a baseline for understanding how terrestrial planets develop during their infancy. Researchers compare the intense surface conditions of modern lava worlds to the primordial stage of our own solar system.
“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,” Coy noted. “We think that very early in the solar system’s history, when the terrestrial planets formed, they were extremely hot due to the energy from all of the planetesimal collisions. Earth had what’s known as a magma ocean stage with an entirely liquid surface. This result gives us a window into studying what conditions may have been like in Earth’s first couple of million years.”