The Hottest Known Planet Continuously Melts its Own Atmosphere

As we peer out into the universe in search of other worlds, we’ve spotted many gas giants orbiting close to their home stars. These so-called “hot Jupiters” have extreme environments, but the planet KELT-9b is in a class of its own. This is the hottest exoplanet ever discovered, and we’re just now understanding what that means. A new analysis of KELT-9b shows that molecules in its atmosphere are torn apart.

Astronomers discovered KELT-9b in 2017 orbiting a star about 670 light-years away. It has a surface temperature of 7,800 degrees Fahrenheit (4,300 degrees Celsius). That makes it the hottest exoplanet by far, and it’s even warmer than some stars. The planet might have become a star itself if not for its low mass, which is just shy of three times that of Jupiter.

The observational data is a good match for computer models developed here on Earth. Without hydrogen dissociation, models predict unfathomable wind speeds of 37 miles per second (60 kilometers per second). Hydrogen dissociation distributes the heat more evenly, although astronomers are still baffled by the location of the planet’s “hot spot,” which isn’t directly under the star as expected. That’s just one more mystery to investigate on KELT-9b.