Astronomers using NASA’s James Webb Space Telescope (JWST) have uncovered the tumultuous history of a distant, hellishly hot exoplanet that’s being stretched and scorched by its star.
The planet, known as WASP-121b, is locked in a dangerously close orbit around a star roughly 900 light-years away that’s brighter and hotter than our sun. Locked in a blistering 30-hour orbit, the world lies so close to its star that intense tidal forces have warped it into a football-like shape, leaving it on the verge of being torn apart by gravity. One side of the planet faces its star permanently, baking at temperatures over 3,000°C (5,400°F) — hot enough for it to rain liquid iron. Even the opposite hemisphere, locked in eternal night, simmers at 1,500°C (2,700°F). This extreme environment makes WASP-121b one of the most hostile planets ever observed, and a valuable target for planetary science.
Now, using the James Webb Space Telescope’s (JWST) Near Infrared Spectrograph instrument, or NIRSpec, a team led by astronomer Thomas Evans-Soma of the University of New Castle in Australia detected a cocktail of molecules in the planet’s atmosphere that each carry chemical clues to its dramatic journey. These include water vapor, carbon monoxide, methane and, for the first time ever in a planetary atmosphere, silicon monoxide.
Together, they tell a dramatic origin story of WASP-121b written in vapor and stone, described in two papers published Monday (June 2).
“Studying the chemistry of ultra hot planets like WASP-121b helps us to understand how gas giant atmospheres work under extreme temperature conditions,” Joanna Barstow, a planetary scientist at the Open University in the U.K. and a co-author of both new studies, said in a statement.
The findings from both studies suggest WASP-121b did not form where it is today. Instead, it likely originated in a colder, more distant region of its planetary system, similar to the zone between Jupiter and Uranus in our own solar system. There, it would have accumulated methane-rich ices and heavy elements, embedding a distinct chemical signature in its growing atmosphere.
Later, gravitational interactions — possibly with other planets — would have sent WASP-121b spiraling inward toward its star. As it moved closer, its supply of icy, oxygen-rich pebbles would’ve been cut off, but it should have been able to continue gathering carbon-rich gas. This would explain why the world’s atmosphere today contains more carbon than oxygen, a chemical imbalance that offers a snapshot of its journey through the disk.
To make sense of the complex atmospheric data, the second team of researchers, led by Cyril Gapp of the Max Planck Institute for Astronomy in Germany, created 3D models of the planet’s atmosphere, accounting for the vast temperature differences between the day and night sides. Their simulations, described in a paper published in The Astronomical Journal, helped separate signals from different regions of the planet as it orbited, revealing how molecules shift and circulate throughout the orbit.
Among the molecules newly detected, the presence of silicon monoxide was particularly revealing, scientists say, as it isn’t typically found in the gaseous form they observed. Instead, the researchers suggest this gas was originally locked in solid minerals like quartz within asteroid-size planetesimals that crashed into the young planet. Over time, as the planet grew and spiraled inward toward its star, those materials would have been vaporized and mixed into its atmosphere, according to one of the new papers, published in Nature Astronomy.
On the cooler “night” side of WASP-121b, the researchers found an abundance of methane gas. This came as a surprise as methane typically breaks down under such heat, the study notes.
“Given how hot this planet is, we weren’t expecting to see methane on its nightside,” study co-author Anjali Piette, who is an assistant professor of astronomy at the University of Birmingham, said in a statement.
Its presence suggests methane is being replenished, likely pulled up from deeper, cooler layers of the atmosphere.
“This challenges exoplanet dynamical models, which will likely need to be adapted to reproduce the strong vertical mixing we’ve uncovered on the nightside of WASP-121b,” study lead author Thomas Evans-Soma of the University of New Castle in Australia added in another statement.
