Researchers Discover a Planet That’s As Hot As a Star

It’s daytime temperatures clock in at a scorching 7,800 degrees Fahrenheit

The star KELT-9 and its hellish planet KELT-9b Robert Hurt / NASA/JPL-Caltech

While our planet is irrefutably warming, it’s unlikely it will every reach the blazing temperatures on KELT-9b, a recently discovered exoplanet whose daytime temperatures soar at over 7,800 degrees Fahrenheit.

The planet lies about 650-light years from Earth in the constellation Cygnus. And it's the hottest discovered so far, writes Ben Guarino for the Washington Post, dwarfing our solar system’s hottest rock, Venus, which has an average high of 860 degrees Fahrenheit. The researchers describe the find in a paper published this week in the journal Nature.

So why is KELT-9b so hot? Because of its star. KELT-9b orbits a fiery body that is twice as large and twice as hot as our own sun, according to a NASA press release. The planet sticks close to its star and, like our own moon, is tidally locked into its orbital dance. This means that one side always faces the star while the other is left in perpetual darkness. 

So much heat and ultraviolet radiation bombards the planet's daytime side that molecules like methane cannot form—elements on that half of the world exist in atomic form. The radiation has also caused the planet's atmosphere to puff up, making KELT-9b much larger than expected. While the planet is 2.8 times as massive as Jupiter, the largest planet in our solar system, it is only half as dense. According to the press release, the constant bombarding radiation may also cause material to continually evaporate from its surface, producing a tail similar to a comet.

“It's a planet by any of the typical definitions of mass, but its atmosphere is almost certainly unlike any other planet we've ever seen just because of the temperature of its dayside," Scott Gaudi, professor of astronomy at Ohio State University who led the study, says in the release. In fact, as Guarino reports, when Gaudi and his colleagues collected the data on the planet, they each bet a bottle of scotch on whether the object would turn out to be a planet or something else. 

But, like most planets, its star is working through its hydrogen fuel. It will eventually run out, causing it to swell up to three times its current size, Gaudi tells Guarino. “It will actually eat the planet. And then what happens — who knows?” he says.

But even before that, the gas giant planet may undergo a radical change. “KELT-9 radiates so much ultraviolet radiation that it may completely evaporate the planet,” Keivan Stassun, an astronomer at Vanderbilt University and collaborator on the study says in a press release. “If gas giant planets like KELT-9b possess solid rocky cores as some theories suggest, the planet may be boiled down to a barren rock, like Mercury.”

According to the press release, KELT-9b will be visible for about 150 more years before it moves out of view for three millennia. Before then, the researchers hope to get some time on the Hubble telescope and other powerful scopes to see if the planet actually does have a “comet tail” and to determine just how long it can survive the hellfire.