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Scientists have discovered an unusual rocky exoplanet that is both hot enough to have a lava-covered surface and unexpectedly cool enough to retain an atmosphere.
The planet, HD 3167 b, is a rocky super-Earth located about 154 light-years from Earth. It orbits its host star at an extremely close distance, completing one revolution in just 24 hours.
Researchers say it is the smallest known lava world with an atmosphere and, surprisingly, the coolest such planet discovered so far with an atmosphere.
The findings were based on observations by the James Webb Space Telescope’s Mid-Infrared Instrument (MIRI), which allowed scientists to study the planet in mid-infrared light and estimate its temperature.
The team observed HD 3167 b during a secondary eclipse, when the planet passed behind its star. By measuring changes in infrared light, researchers were able to estimate the planet’s thermal properties and look for evidence of an atmosphere.
The planet is tidally locked, meaning one side permanently faces its star while the other remains in darkness. Given its extremely close orbit, scientists expected its dayside to reach temperatures close to the theoretical maximum for a rocky planet without an atmosphere.
Instead, the observations showed that the planet is significantly cooler than expected, suggesting that it has an atmosphere capable of redistributing heat.
Researchers believe the atmosphere reflects some of the incoming starlight and transports heat from the intensely heated dayside toward the planet’s cooler nightside.
“What’s so surprising is that the closer a rocky planet orbits its star, the harder it should be to have an atmosphere,” said Edwin Kite, an associate professor of geophysical sciences at the University of Chicago and a co-author of the study. He noted that such planets are exposed to intense stellar winds and high-energy radiation.
The discovery adds to a growing body of evidence that even extremely hot lava worlds may be able to retain atmospheres.
Scientists are now investigating how such atmospheres form and survive under extreme conditions. A broader research programme led by astronomer Megan Weiner Mansfield of the University of Maryland is studying 10 lava worlds to determine how temperature affects their ability to maintain atmospheres.
Although HD 3167 b is far too hostile for life, researchers say studying such planets could provide clues about the early history of rocky planets, including Earth.
“Very early in the solar system’s history, when the terrestrial planets formed, they were extremely hot,” said Brandon Park Coy, the study’s lead author and a graduate student in Kite’s group.
Earth itself is thought to have passed through a magma-ocean stage, when much of its surface was molten. Studying modern lava worlds could therefore offer scientists a glimpse into conditions that existed during the first few million years of Earth’s history.