A planet with an atmosphere where water molecules glow is the strongest evidence yet of the first stratosphere discovered on a world outside the Solar System. Using data from NASA's Hubble Space Telescope, an international team of scientists led by the University of Exeter has found that the upper atmosphere of exoplanet WASP-121b is as hot as boiling iron, indicating that it's stratified in a manner similar to Earth.

Discovered in 2015, WASP-121b is a superhot gas giant 1.18 times the mass of Jupiter orbiting the star WASP-121, which is about 900 light years from Earth in the constellation of Puppis. The "hot Jupiter" revolves about its star every 1.27 days and is almost within the Roche limit of WASP-121, where the tidal forces would tear the planet apart.

But what interests planetologists about WASP-121b is its atmosphere. Analysis shows that the light spectrum changes dramatically with altitude to the point where, instead of absorbing light, the water molecules start to glow and emit infrared light as the molecule's electrons move into a higher energy state and then revert. This indicates that the temperature of the upper layer increases the higher one goes, which is the defining characteristic of the stratosphere.

According to the Exeter team, the upper atmosphere reaches temperatures as high as 2,500° C (4,500° F), which is near the boiling point of iron. On Earth and other planets and moons in the Solar System, the stratosphere is caused by the ultraviolet radiation from the Sun being trapped by ozone or methane, though the temperature rise is much less dramatic by a factor of ten.

Theoretical models suggest that WASP-121b may be one of a special class of ultra-hot exoplanets. It's possible that vanadium oxide and titanium oxide gases may be responsible for the temperatures because they can strongly absorb light at visible wavelengths. These compounds are often found in the spectra of brown dwarf stars, which have many properties in common with super Jupiters.

"We've measured a strong rise in the temperature of WASP-121b's atmosphere at higher altitudes, but we don't yet know what's causing this dramatic heating," says Nikolay Nikolov, research fellow at the University of Exeter. "We hope to address this mystery with upcoming observations at other wavelengths."

The research was published in Nature.

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