We’ve just found the hottest planet ever

Astronomers with the Kilodegree Extremely Little Telescope(s) (KELT) survey have just announced an amazing find: the hottest gas giant ever discovered. In fact, the planet is so hot that it’s hotter than most stars, and it’s only a few thousand degrees cooler than our own Sun.


The planet, KELT-9b, is about three times the mass of Jupiter and twice its size. Its discovery was announced by B. Scott Gaudi of The Ohio State University and Karen Collins of Vanderbilt University at a press conference Monday afternoon at the 230th Meeting of the American Astronomical Society in Austin, Texas. “We are very excited today to announce the discovery of KELT-9b … a gas giant planet hotter than most stars. And I want to emphasize that’s not a typo,” Collins said during the press conference. She went on to explain that KELT-9b is so hot because of its sun, “the brightest, hottest, most massive known transiting gas giant planet host star.” Concurrent with the announcement, the work was also published online as a letter in Nature.


That host star is roughly 2.5 times the mass of our Sun, and is rotating so quickly (about once a day) that it’s more of a flattened egg shape than a sphere, like the planet Saturn. Every time KELT-9b transits across the face of its sun, the light coming from the star drops by only one half of one percent. The star, which is a hot, blue star, radiates not only in the optical, but also puts out huge amounts of ultraviolet (UV) light. Its massive output, coupled with KELT-9b’s close proximity, boosts the temperature on the planet’s day side to about 7,800 degrees Fahrenheit (4,300 Celsius). Because the planet is tidally locked, the same side always faces its parent star; astronomers currently think that the night side is much cooler, due to the atmosphere’s poor ability to transfer heat from the broiling day side to the rest of the planet. But even still, that “cool” side is hot: “The night side would probably look like a red dwarf to our eyes,” Gaudi said at the press conference.


Furthermore, KELT-9b is orbiting its star perpendicular to the host’s axis of rotation. That means rather than circling in the same plane as the star’s equator, as our planets circle the Sun, KELT-9b flies over its parent star’s north and south poles with every 1.5-day orbit it completes. This odd orbit, Gaudi said, likely precesses as well, which means the planet may stop transiting its sun as seen from Earth within about 150 years, depending on the rate of this precession. Astronomers would then have to wait several thousand years before transits could be seen again.

KELT-9b orbits perpendicular to its star’s rotation axis, meaning it swings over its sun’s north and south poles, rather than circling its equator.


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