Kelt-9b: A hot Jupiter Hotter Than Most Stars

A newly discovered Jupiter-like world is so hot that it's stretching the definition of the word "planet."

With a day-side temperature of 4,600 Kelvin (more than 7,800 degrees Fahrenheit), planet KELT-9b is hotter than most stars, and only 1,200 Kelvin (about 2,000 degrees Fahrenheit) cooler than our own sun.

In this week's issue of the journal Nature and at a presentation at the American Astronomical Society spring meeting, an international research team led by astronomers at The Ohio State University and Vanderbilt University describes a planet with some very unusual features.

For instance, it's a gas giant 2.8 times more massive than Jupiter but only half as dense, because the extreme radiation from its host star has caused its atmosphere to puff up like a balloon. And because it is tidally locked to its star--as the Moon is to Earth--the day side of the planet is perpetually bombarded by stellar radiation, and as a result is so hot that molecules such as water, carbon dioxide, and methane can't form there. The properties of the night side are still mysterious--molecules may be able to form there, but probably only temporarily.

"It's a planet by any of the typical definitions based on mass, but its atmosphere is almost certainly unlike any other planet we've ever seen just because of the temperature of its day side," said Scott Gaudi, professor of astronomy at The Ohio State University and a leader of the study.

KELT-9b orbits a star, dubbed KELT-9, which is more than twice as large and nearly twice as hot as our sun. Keivan Stassun, a professor of physics and astronomy at Vanderbilt who directed the study with Gaudi said, "KELT-9 radiates so much ultraviolet radiation that it may completely evaporate the planet. Or, if gas giant planets like KELT-9b possess solid rocky cores as some theories suggest, the planet may be boiled down to a barren rock, like Mercury."

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