Astronomers using the James Webb Space Telescope have discovered HD 3167 b, the smallest and coldest lava world ever found with an atmosphere. Orbiting a star 154 light-years away every single Earth day, this rocky super-Earth defies expectations by retaining an atmosphere that cools its dayside, offering a rare window into Earth’s primordial magma ocean stage.
Detecting the Coldest Lava World with the James Webb Space Telescope
Since the first exoplanet was confirmed over 30 years ago, science has mapped more than 6,300 distant worlds. These discoveries range from massive gas giants resembling Jupiter to Neptune-like bodies. Yet, a rare subset of these discoveries remains intensely compelling: lava worlds.
These are rocky planets where surface temperatures are hot enough that their surfaces are melted into burning lava. Until now, finding an atmosphere on such a world seemed structurally improbable.
That paradigm shifted when researchers published a new study in The Astrophysical Journal Letters detailing observations of HD 3167 b. The research team utilized the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope to capture the planet during a secondary eclipse.
By measuring minute shifts in mid-infrared light as the planet slipped behind its host star, the team calculated thermal radiation profiles. They expected a bare rock exposed to such intense stellar proximity to exhibit maximum theoretical heat on its dayside.
Instead, the data revealed an anomaly. HD 3167 b is noticeably cooler than its expected maximum. The planet features a tidally locked orbit, meaning one permanent dayside faces the star while the other remains in perpetual darkness.
Lead author Brandon Park Coy, a graduate student in the research group led by University of Chicago associate professor Edwin Kite, pointed out the physical discrepancy. The planet’s atmosphere actively cools its dayside by reflecting stellar radiation and redirecting thermal energy toward the nightside.
Co-author Edwin Kite noted the fundamental difficulty these worlds face. The closer a rocky planet orbits its host star, the more violently stellar winds and high-energy photons bombard it, stripping away volatile gases. Yet, HD 3167 b and a handful of other extreme worlds manage to retain their gaseous envelopes.
An Observational Campaign Across Ten Extreme Exoplanets
This finding does not stand alone. It anchors a broader investigative effort designed to map out the exact thermal boundaries where rocky exoplanets can successfully hold onto an atmosphere.
Astronomer Megan Weiner Mansfield at the University of Maryland leads this targeted program. The initiative examines a cohort of 10 distinct lava worlds to establish empirical thresholds for atmospheric retention.
Data gathered from MIRI observations challenge long-held assumptions regarding planetary formation and atmospheric survival. While scientists have previously documented atmospheres on ultra-hot lava worlds, finding one on a smaller, relatively cooler body upends theoretical models.
It forces astrophysicists to reconsider how these volatile envelopes form, how long they persist under heavy stellar bombardment, and what chemical mechanisms shield them from total erosion.
A Direct Window Into Earth’s Magma Ocean Infancy
While extreme lava worlds are entirely hostile to biological life, their utility to planetary science is immense. They serve as natural laboratories for studying physical processes that govern all terrestrial worlds.
Researchers are particularly interested in these harsh environments because Earth’s own history likely began in a remarkably similar state. During the terrestrial planet formation phase in our solar system’s infancy, intense kinetic energy from frequent planetesimal collisions melted the surface.
During this primordial epoch, Earth possessed a magma ocean stage with a completely liquid exterior. Studying HD 3167 b grants researchers an empirical proxy for conditions that prevailed during Earth’s first couple of million years.
By bridging observation with geophysical modeling, astronomers can better understand the atmospheric dynamics that allowed our own planet to cool, stabilize, and eventually foster conditions capable of supporting life.
