Astronomers have discovered the hottest gas giant exoplanet yet: a Jupiter-like world that is hotter than most stars.


KELT-9b is 2.8 times more massive than Jupiter, but is only half as dense.

This is because extreme radiation from the host star, KELT-9, has caused the planet to expand like a balloon.

KELT-9b is tidally locked, meaning the same side of the planet always faces the star as it orbits around it.

This leaves one side of the planet in perpetual daylight, clocking up a temperature of about 4,315°C and making it hotter than most stars.

KELT-9 also orbits perpendicular to the star’s spin axis, which would be the equivalent of a planet orbiting perpendicular to the plane of our Solar System.

”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," says Scott Gaudi, astronomy professor at The Ohio State University in Columbus, who led a study on the planet.

The KELT-9 star is just 300 million years old, yet is over twice as large and nearly twice as hot as our Sun.

This extreme heat and radiation may be causing a comet-like tail to form on the planet, and could eventually vaporise it entirely.

That is, provided the star doesn’t swallow it first.

"KELT-9 will swell to become a red giant star in a few hundred million years," says Keivan Stassun, who directed the study with Gaudi.

"The long-term prospects for life, or real estate for that matter, on KELT-9b are not looking good."

The hot Jupiter exoplanet was found using the transit method, whereby astronomers look for regular dips in the brightness of a star, which may indicate a planet in orbit around it.

"As has been highlighted by the recent discoveries from the MEarth collaboration, the planet around Proxima Centauri, and the astonishing system discovered around TRAPPIST-1, the astronomical community is clearly focused on finding Earthlike planets around small, cooler stars like our Sun,” says Gaudi.

“They are easy targets and there's a lot that can be learned about potentially habitable planets orbiting very low-mass stars in general.


On the other hand, because KELT-9b's host star is bigger and hotter than the Sun, it complements those efforts and provides a kind of touchstone for understanding how planetary systems form around hot, massive stars.”


Iain Todd, BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.