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 it does not rotate around its own axis, just as the moon does not spin while it orbits 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-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 vapour 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 hypothesise 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 emphasised 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.
