How hot is it? The hottest-known exoplanet is so hot it rips apart its own molecules

The host star is a hot, rapidly rotating A-type star that is about 2.5 times more massive and almost twice as hot as our sun. The hot star blasts its nearby planet KELT-9b with massive amounts of radiation, leading to a daylight temperature of 7800 degrees Fahrenheit, hotter than most stars and only 2000 degrees cooler than the sun.

KELT-9b is a giant exoplanet hotter than some stars, reaching 7,800 degrees Fahrenheit on its surface. Yet, it somehow has an atmosphere. But a new study using data from NASA's Spitzer Space Telescope has revealed that the molecules in its atmosphere are constantly ripped to pieces.

The exoplanet was initially discovered in 2017 by astronomers using the Kilodegree Extremely Little Telescope project, which combines the observation power of two robotic telescopes in South Africa and southern Arizona.

It's what astronomers refer to as an ultra-hot Jupiter. Hot Jupiters were some of the first exoplanets to be discovered. Although they're in orbits too close to their stars to support life on the planet's surface, these intriguing gas giants are unlike any planet in our solar system.

KELT-9b is a rarity, even in its own class of exoplanets. It's in the Cygnus constellation, orbiting a star 670 light-years from Earth. It clocks in at three times the mass of Jupiter and is so close to its star, KELT-9, that it completes on orbit around it every 1.5 Earth days.

The planet is tidally locked to its star, like how the moon always shows the same face to Earth. And new data provided by Spitzer, NASA's infrared observatory, showed that hydrogen gas molecules are shredded on the dayside of the planet. Those molecules can't reform until they appear on the nightside of the planet, which is slightly cooler.

The process repeats when the unfortunate molecules return to the dayside. The findings were published this week in the journal Astrophysical Journal Letters.

"This kind of planet is so extreme in temperature, it is a bit separate from a lot of other exoplanets," said Megan Mansfield, lead study author and a graduate student at the University of Chicago. "There are some other hot Jupiters and ultra-hot Jupiters that are not quite as hot but still warm enough that this effect should be taking place."

Spitzer is sensitive to variations in heat, so observations by the telescope allowed the astronomers a chance to detect the temperature flux between the day and nightsides. In this case, the temperature difference isn't a large one, meaning that heat likely flows from the dayside to the nightside.

But a new twist suggested by the data is that the planet's "hot spot," which should be directly beneath the glare of the star, has shifted, according to the study. KELT-9b continues to surprise researchers.

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