A tidally locked planet of 7,800 degrees Fahrenheit and its star. (R. Hurt (IPAC), NASA/JPL-Caltech)

The hottest planet ever discovered orbits a massive star so sizzling it burns bluish white. On exoplanet KELT-9b, a gas giant 650 light-years from Earth, temperatures during the day reach 4,300 degrees Celsius, or 7,770 degrees Fahrenheit. That's only about 1,300 degrees Celsius cooler than the surface of the sun. Venus, the planet in our solar system where temperatures regularly are the hottest, averages a high of about 460 degrees Celsius at ground level.

“KELT-9b is one of the strangest exoplanets I’ve ever seen,” said Scott Gaudi, a professor of astronomy at Ohio State University who has been searching for exoplanets for the past two decades.

On Monday, Gaudi and his co-authors reported in the journal Nature that KELT-9b's incredible heat is the result of its unusually close relationship with its unusually hot star. The data from the hot exoplanet were so strange that there was a bet between Gaudi and another scientist, involving a bottle of single-malt scotch, whether KELT-9b was, in fact, a planet. (Gaudi won.)

In terms of its atmosphere, KELT-9b is like a hybrid between a star and a planet, said Drake Deming, a University of Maryland astronomer not involved with this study, though he cautioned that calling KELT-9b a star-planet hybrid was “somewhat of an oversimplification.” KELT-9b is as hot as a star — warmer, in fact, than some — but unlike a star, the planet does not have a nuclear core that fuses hydrogen into helium.

Astronomers spotted the planet three years ago, using a pair of instruments named the Kilodegree Extremely Little Telescopes, or KELT. As KELT-9b moved in front of the star, the star's brightness dimmed. The dips occurred every 36 hours. Put another way, KELT-9b is so close to its sun that it completed a yearly orbit in 1.5 Earth days.

KELT-9b is an odd world. All of the planets in our solar system orbit the sun's equator. But KELT-9b orbits around the poles of its massive star. KELT-9b is tidally locked, too, which means the planet does not appear to spin on its axis as it orbits the star, in the way that the same face of the moon always faces Earth. (The hottest temperatures, then, are found in KELT-9b's dayside atmosphere, the half of the planet facing its star.)

Though scientists had previously discovered “hot Jupiter” planets, so named for their extreme temperatures and gas giant nature, all fell about a thousand degrees Celsius short of KELT-9b. KELT-9b is at the “extreme end of the population” of known gassy, hot giants, said Jonathan Fortney, a University of California at Santa Cruz professor who creates computer models to determine what's in an exoplanet atmosphere.

Fortney, who was not directly involved with this paper, was asked to model KELT-9b's atmosphere. He declined. “I can’t possibly do that. The planet’s way, way too hot,” he said.

A planet's atmosphere is typically made up of molecules, which are made up of atoms bound together. Molecular hydrogen gas is common on Jupiter, for instance, as are water vapor and other gases on Earth. KELT-9b's dayside is an exception. The heat breaks molecules into its constituent atoms. “Almost all the elements are in atoms, not in molecules,” Fortney said. “It's essentially too hot for molecules to exist.”

Gaudi and his colleagues hypothesize that KELT-9b may even have a tail, like a comet, as the massive star blasts away at the planet; it could expose naked rock, leaving behind something like Mercury. Deming emphasized that this hypothesis was unproven. “We don't know enough” to suggest that radiation is stripping KELT-9b's atmosphere, he said.

Every planet's days are numbered, but KELT-9b is particularly precarious. As the KELT-9 star runs out of hydrogen, it will swell and cool to three times its current size, Gaudi said. “It will actually eat the planet. And then what happens — who knows?”

But, given the millions of years in the meantime, this will not be the last we hear about KELT-9b. When asked what about the KELT-9 system Gaudi is looking forward to studying, he said, “Everything.” Astronomers are lobbying to point other instruments, such as the Hubble telescope, at the planet, to see if, in fact, it has a comet-like tail. They will also attempt to discern more precisely what makes up KELT-9b's atmosphere.

Most exoplanet hunters are drawn to smaller, cooler stars, because that's where possibly habitable worlds could exist. But this means that very large, very hot stars are neglected, Gaudi said. Around these stars, though, nature becomes truly bizarre. “It follows the trend of things getting weirder and weirder the more you look.” 

Correction: A previous version of this article incorrectly stated that KELT-9b does not rotate on its axis. It does, but relative to its star it does not appear to do so.

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