The Hottest Planet Ever Was Just Found, and is Likely Evaporating

by Monica Hunter-Hart

Astronomers are baffled by the discovery of an extremely hot exoplanet that in many ways isn’t like a planet at all. The 300 million-year-old “KELT-9b” is young in space terms, but may not be around for long.

At 7,800 degrees Fahrenheit, KELT-9b is hotter than any known planet and most stars. But that’s nothing compared to the 17,900-degree heat of its host star, which is so hot that scientists are pretty sure it’s evaporating KELT-9b.

It “radiates so much ultraviolet radiation that it may completely evaporate the planet,” Keivan Stassun, director of a study on KELT-9b published in Nature on Monday, told NASA.

KELT-9b may escape that fate, but only because the star — which is growing, and will eventually become a red giant — will probably engulf it first. “The long-term prospects for life, or real estate for that matter, on KELT-9b are not looking good,” Stassun says.

That’s too bad, because KELT-9b is pretty amazing; in addition to its star-like heat, it may trail evaporating planetary material behind it like a comet. KELT-9b does meet the qualifications for being a planet, but it’s an oddity.

KELT-9b also has an unusually large radius for its mass, probably because radiation from the host star has caused its atmosphere to balloon outward. Unlike any planet in our solar system, its orbit runs perpendicular to the star’s rotation.

An artist imagines KELT-9b orbiting its star.


KELT-9b is tidally locked to its star, meaning that it only ever faces the star on one side, like how we only see one side of the moon from Earth. This makes for an uneven heat distribution: The orange dayside is so hot that the hydrogen and helium atoms can’t even bond together to form molecules, while the dark red nightside (which experiences constant darkness) may allow molecules to at least temporarily form. “The transition from the dayside to the nightside might be very interesting,” the study’s coauthor Scott Gaudi tells Inverse, “with molecules reforming as gas moves from the hotter dayside to the cooler nightside.”

The planet is way too hot to be habitable by humans, even if it weren’t doomed. Still, it’s an important discovery; Gaudi told CNN that a discussion about colonizing other planets “should take place in the context of understanding how planetary systems form,” and scientists don’t yet understand how this incredibly hot system was created.

KELT-9b “demonstrates that giant planets can indeed form around very massive stars, and that these planets can migrate to very close in orbits relatively early in the life of those stars,” Gaudi tells Inverse.

Hopefully they’ll soon gain more insight into KELT-9b’s quirks. Until then, this exoplanet weirdo will be whirling up there on the road less traveled by, like a darn individual.

Related Tags