Scientists using the James Webb Space Telescope (JWST) have made new discoveries about the origin and atmosphere of a distant planet called WASP-121b.
This planet is extremely hot and orbits its star in just 30.5 hours—so close that one side is always facing the star with scorching temperatures above 3000°C, while the other side remains in constant darkness at about 1500°C.
Researchers detected several key gases in the planet’s atmosphere, including water vapor, carbon monoxide, silicon monoxide, and methane.
These findings give scientists clues about where and how the planet formed.
Interestingly, the presence of methane on the cooler “nightside” of the planet was a surprise, because current scientific models didn’t expect it to exist in such large amounts.
By studying these gases, the scientists—led by Thomas Evans-Soma from the Max Planck Institute and Cyril Gapp, a student at the same institute—were able to estimate the amounts of carbon, oxygen, and silicon in the atmosphere.
They believe that WASP-121b was originally formed far from its current location, in a colder part of its solar system where water stayed frozen but methane could turn into gas.
This is similar to the region between Jupiter and Uranus in our solar system. After forming, the planet likely moved much closer to its star over time.
The presence of silicon monoxide is also important. Silicon likely came from rocky material such as quartz that was pulled in by the planet after it had already collected most of its gas. This shows that the planet continued growing in stages, collecting different materials over time.
Planet formation starts with small icy dust grains sticking together to form pebbles, which eventually become planet-sized bodies. These pebbles carry gases and ices with them as they spiral toward the star.
Depending on where in the disk the planet forms, it can absorb different materials. For WASP-121b, it seems the planet formed in a region where methane was available as gas, while water remained frozen.
This would explain why the planet has more carbon than oxygen in its atmosphere—carbon-rich gases kept flowing in, while oxygen-rich ice pebbles were no longer available.
The discovery of methane on the planet’s nightside was unexpected because methane normally breaks apart in high heat, and scientists assumed it wouldn’t be present even on the cooler side.
But its strong presence suggests something unusual is happening: powerful vertical winds are likely bringing methane up from deeper, cooler layers of the atmosphere. This challenges what scientists thought they knew about how gases move around on hot planets.
The team used JWST’s Near-Infrared Spectrograph (NIRSpec) to follow the planet through its full orbit and during its transit across the star. This allowed them to study both the hot and cool sides of the planet, as well as the area in between. The telescope’s powerful instruments confirmed the presence of several gases and gave scientists a clearer picture of the planet’s complex and dynamic atmosphere.
These findings not only reveal how WASP-121b may have formed and migrated but also raise new questions about how atmospheres behave on extremely hot worlds.
