More info i
Exoplanet KELT-9b. Credit: NASA

Some planets are more scorching than stars, because chemistry can get weird out there

Contributed by
Apr 1, 2021, 2:12 PM EDT

Hot Jupiters are gaseous beasts that can orbit their stars at distances we Earthlings would consider terrifyingly close. They don’t stop at just “hot,” either. There are ultra-hot Jupiters and — the mother of them all — extremely hot Jupiters.

Enter KELT-9b. At 7,800 degrees Fahrenheit, it is hotter than most known stars. Most hot Jupiters are hell with air conditioning, while KELT-9b is the unmitigated fire of the inferno. The extremes of this planet even make scientists sweat. Now, new research has revealed another something else hot about it. The molecules of metal vapor found in their atmospheres shouldn’t be there. They can’t take the heat. But they are, which could tell us more about what happens on this planet and even how it came into being.

The survival and raging heat of KELT-9b are probably the result of its strange chemical makeup along with a hypothetically turbulent formation that involved collisions with other objects as it moved toward the fireball it orbits.

“KELT-9b is particularly interesting because it's unexpectedly close to its star and has a very inclined orbit,” said Quentin Changeat, who co-authored a study recently published in The Astrophysical Journal Letters. “It suggests some major event happened in the past, so it is a very good candidate to look for these unexpected chemistries.”

Obviously, the only thing hot enough to have raised the body temperature of KELT-9b so high is its host star. Earth couldn’t possibly take the pressure that KELT-9b experiences. Its atmosphere is a lethal cauldron of H2, metal oxides and metal hydrides, which could never occur in our cosmic territory. How those molecules were even floating around in the atmospheres was a mystery, since so much heat is supposed to dismember them. The thing is that only one side of the planet is going through a perpetual hot flash.

KELT-9b in orbit, slowly being vaporized by its star. Credit: NASA


Because it is tidally locked, much like our Moon, KELT-9b does get relief on its night side. This is what gave Changeat an idea of what might be happening so those molecules can at least temporarily keep it together. The heat on the day side may destroy the molecules, but not the atoms they are made of. If winds sweep those atoms to the much cooler night side, and they bond again to form the same types of molecules they did before, that can explain why they were detected in an atmosphere they otherwise wouldn't make it out of alive.

“Where this becomes interesting is that those metal oxides and hydrides are supposed to be thermally dissociated into smaller atomic and ionic species at those temperatures,” Changeat said. “So this suggests some important physical processes are going on here, and that our traditional predictions for this planet may not be so accurate, but it is still very early to say.”

This can start to demystify something else about KELT-9b. It is thought to take intense physical processes for a gas giant to form and orbit so close to its star without being instantly vaporized. Understanding the formation and evolution of KELT-9b might give better insight as to how it’s been able to survive for at least hundreds of millions of years, and where a planet formed and how it evolved, along with any collisions it might have faced along the way, can be revealed by what makes up its atmosphere.

Finding out how much of those metals are in KELT-9b’s atmosphere can potentially tell us where it came from and how it got to where it is. The intact metal molecules that shouldn’t have survived showed themselves in Hubble observations of KELT-9b in eclipse mode, when it was completely hidden behind its star. Spectral features associated with those metals gave them away. But where did they come from in the first place if the planet is so inhospitable to them? Changeat believes that they might not have originated in the danger zone.

“It is still difficult at this stage to exactly say why, but some hypothesis could be that they are from the cooler regions, like the night side or deeper atmosphere, and are displaced into the day side where we can see them,” he said. “There are probably other processes involved, but we would most likely need another telescope to know more.”

Because Hubble, which is a relic of the ‘90s, isn’t equipped to study exoplanet atmospheres, next-gen telescopes like James Webb and Ariel will be able to see more than ever. Ariel will observe a thousand exoplanet atmospheres and transmit data about their physical and chemical properties back to Earth. It better hurry. Though the KELT-9 star didn’t annihilate KELT-9b when it ventured too close, the planet is thought to be gradually vaporizing as it orbits, with a comet-esque tail of gas and dust trailing behind it as it slowly succumbs to the heat.

To think, Freddie Mercury thought 200 degrees was hot enough to be called “Mr. Fahrenheit”, but in his defense, there was no proof exoplanets existed back then.