The James Webb Space Telescope has revealed evidence of an atmosphere around the rocky planet HD 3167 b, a surprising finding for an extremely hot world that was expected to be stripped of its gases by radiation and stellar winds.
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The planet is located about 154 light-years away, orbits a K-type star (orange in color) in the constellation Pisces, and completes its orbit in just one Earth day.
The significance of the discovery stems from the fact that HD 3167b has become the coldest known lava world to show evidence of an atmosphere, giving scientists a natural laboratory to understand the ability of extreme rocky planets to retain gases, and perhaps to reconstruct an early picture of Earth itself when its surface was molten.
HD 3167b is classified as a "super-Earth," a rocky world larger than Earth but smaller than giant planets like Neptune. A "lava world," on the other hand, is a planet so close to its star that its surface becomes hot enough to melt parts of it.
Scientists believe that HD 3167b is composed of silicates, materials also found in Earth's mantle. The atmospheres of distant rocky planets are a key target for astronomers, as evidence of atmospheric gases was found in only a limited number of the more than 6,300 exoplanets known at the time of the study.
Less heat reveals what telescopes cannot see.
The researchers used the "secondary eclipse method," which measures the slight change in light when a planet disappears behind its star. By observing mid-infrared radiation, the James Webb Space Telescope was able to measure the heat emitted from the planet's dayside.
If HD 3167b were a bare rock without an atmosphere, its temperature would have been expected to reach a higher level determined by the very short distance between it and its star and the properties of its surface, but measurements showed that it is colder than this expected limit.
Researchers believe this temperature difference provides evidence of atmospheric processes that alter energy distribution. The study's first author, University of Chicago researcher Brandon Park Cowie, explained that the atmosphere can transfer energy between the planet's day and night sides, and clouds can reflect some of the star's radiation and lower the surface temperature, but scientists have not yet determined the chemical composition of this atmosphere.
While it was expected that rock vapors would dominate the atmospheres of these worlds, recent studies suggest the possibility of heavier gases such as carbon dioxide, carbon monoxide, and water vapor.
A window onto the Earth when it was an ocean of magma
The discovery of an atmosphere does not mean that HD 3167b is habitable; its extreme temperatures make conditions on its surface very harsh. However, its scientific value lies in its potential to help us understand the evolution of rocky planets when their surfaces are molten.
Subsequent observations will attempt to determine the gases surrounding it and how they interact with its extremely hot surface, which may reveal how some planets that are very close to their stars retain their atmospheres despite an environment that should help them lose them.
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