Hot, hot planet sets sizzling new record

This odd exoplanet is hotter than many stars, astronomers say

Planet KELT 9b, the giant illustrated here facing its sun, is the hottest planet astronomers have discovered to date.

R. HURT/IPAC/JPL-CALTECH/NASA

A newly discovered planet is shattering records for being hot. This big ball of gas is actually hotter than many stars, astronomers say.

Called KELT 9b, it’s located some 650 light-years from Earth. A bit like Jupiter, it’s a gas giant. It orbits its host star every 36 hours. But only one side of the planet faces its sun, just as one side of our moon always faces Earth.

It’s “pretty much something out of a science fiction novel,” says Scott Gaudi. An astronomer at Ohio State University in Columbus, he helped discover this unusual exoplanet.

Temperatures on KELT 9b’s dayside, which faces its star, hover at about 4,300Celsius (7,772o Fahrenheit). Jupiter, by comparison, is a chilly -145 oC (-234 oF). That sizzling temp is more than 700 degrees more blistering than the next-hottest exoplanet known. KELT 9b is so hot, in fact, that atoms in its atmosphere cannot bind together to form molecules!

“It’s like a star-planet hybrid,” concludes Drake Deming. He is a planetary scientist at the University of Maryland in College Park and was not involved in KELT 9b’s discovery. He says this is “a kind of object we’ve never seen before.”

One of its oddities: KELT 9b doesn’t orbit around the equator of its sun. Instead, it swoops around its star’s poles. Because the planet orbits so close, it receives an intense blast of radiation from that star. Indeed, that “stellar wind” is so intense that it blows the planet’s atmosphere out like a comet tail. One day, it may eventually strip the planet’s atmosphere away completely.

The planet is so bizarre that it took scientists nearly three years to convince themselves it was real, Gaudi says. And, adds Deming, the bizarre planet is probably not alone out there. He suspects KELT 9b is “the tip of the iceberg” for an undiscovered population of scalding-hot gas giants.

Gaudi’s team described the planet online June 5 in Nature.

Maria Temming is the staff reporter for physical sciences, covering everything from chemistry to computer science and cosmology. She has bachelor's degrees in physics and English, and a master's in science writing.

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