Planetary Sciences News

Meet KELT-9b, the Hottest Exoplanet Ever Discovered

The exoplanet’s host star blasts it with so much radiation that it will someday evaporate.


KELT-9b may just be the weirdest exoplanet astronomer Scott Gaudi has ever found. Gaudi, a researcher at The Ohio State University in Columbus, is the lead author of a paper published today in Nature describing the newly discovered hot, gassy exoplanet that is 3 times the size of Jupiter and located 650 light-years away from Earth.

The newly found exoplanet is tidally locked to its host star, meaning one side eternally faces a blast of radiant heat, Gaudi said. The 4300°C temperature of KELT-9b’s “dayside” is only about 1000°C cooler than the surface of our Sun.

If all of KELT-9b were this hot rather than just the dayside, it could be a star. But it still falls short of that classification because it doesn’t heat itself by means of hydrogen fusion. Instead, its heat comes from the intense radiation from its nearby host star, KELT-9.

exoplanet orbit
(top) An animation showing the pole-to-pole orbital path of KELT-9b. Normally, planets orbit a star’s equator, which causes blips to occur in the red end and the blue end of the star’s color spectrum from the scientists’ point of view. (bottom) In this case, researchers saw a blip only in the middle of the color spectrum, shown in the spectral line. Studying this middle blip revealed the exoplanet’s unusual orbit. Credit: Marshall C. Johnson

KELT-9 is about twice the mass, size, and temperature of our Sun and, sizzling at 9900°C, is the hottest star known to host a transiting exoplanet. If a naked-eye observer could see KELT-9 from Earth, the star would look bright blue because of its high temperature, Gaudi said, rather than the yellow orange of our much cooler Sun.

The star’s tumultuous radiation could be heating up and stripping away KELT-9b’s atmospheric hydrogen and helium and flinging it into a tail like a comet’s, Gaudi said. In fact, the planet may lose mass at a rate anywhere between 10 billion and 10 trillion grams per second because of this heat. In 300 million years, most of the planet could evaporate and potentially leave behind a rocky core, he continued.

The weirdness doesn’t end there. Normally, we think of planets orbiting around a star’s equator. Exoplanet hunters can spot such orbits by periodic blips in the light coming from the star, signaling a planet transiting in front of the star.

However, KELT-9b’s transit was strange because its effect on the star’s Doppler shift—how its light moves from the red end of the spectrum to the blue end of the spectrum as it turns toward the viewer and away from the viewer—was unusually weak for an exoplanet. This weakness is because KELT-9b is in a polar orbit around its host star, Gaudi explained, which makes the orbit look vertical from Earth’s point of view.

Because KELT-9 itself rotates so fast—it takes about a day to complete a rotation, whereas our Sun takes about a month—the star’s gravity tugs on the planet’s pole-to-pole orbital path around the star, swiveling the exoplanet’s orbit around the star’s rotational axis. Over the next few hundred years or more, this tugging could move the planet so that it no longer passes in front of the star from our point of view but instead orbits around its edges. Without a star lighting the background, KELT-9b would then seem to disappear into the murky background of space.

At that point, “we’ll have to wait a couple millennia until it comes back again” to an orbit passing across the star’s face in our line of sight, Gaudi said.

The new observations will help scientists better understand the wide variety of exoplanets capable of orbiting distant stars. “KELT-9b shows us there’s a lot of weird [stuff] out there, and Mother Nature is way more imaginative than we are,” Gaudi said.

—JoAnna Wendel (@JoAnnaScience), Staff Writer

Citation: Wendel, J. (2017), Meet KELT-9b, the hottest exoplanet ever discovered, Eos, 98, Published on 05 June 2017.
© 2017. The authors. CC BY-NC-ND 3.0