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.

Naturally, KELT-9b has sparked interest among astronomers. Of course, KELT-9b cannot support life as we know it, but studying an extreme environment like this can help us better understand all planets. Recent observations with the Spitzer infrared space telescope revealed what these extreme temperatures mean for the atmosphere. Spitzer can measure variations in heat, and it was able to take the temperature of the two halves of KELT-9b as it zipped around the star.

Based on the Spitzer data, astronomers are confident that molecules on the dayside of the planet are continuously ripped to shreds. Not even molecular hydrogen, the smallest and simplest of molecules, has any hope of survival. KELT-9b orbits very close to its star — a year there is just 1.5 Earth days long. Like most exoplanets in such orbits, it’s tidally locked with one side always facing the star. The temperatures there are high enough to disassociate molecules, but the night side is about 50 percent cooler. That’s enough for molecules to reform as they circulate to the opposite side. The result is a cycle of melting and reforming molecules in the clouds of KELT-9b, like some sort of hellish molecular conveyor belt.

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.

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