This Planet Is so Hot the Molecules in Its Own Atmosphere Get Ripped Apart

KELT-9b is the hottest exoplanet yet discovered, with surface temperatures exceeding those of some stars. Now, researchers have found that the planet's atmosphere is so hot, the molecules within it are literally being ripped apart.

The planet, which lies around 670 light-years away, is tidally locked, meaning that the same side is always facing its star. On this dayside, molecules of hydrogen are torn to shreds, before flowing round to the night side where they reform again, according to a study published in the Astrophysical Journal Letters.

"This kind of planet is so extreme in temperature, it is a bit separate from a lot of other exoplanets," Megan Mansfield, a graduate student at the University of Chicago and lead author of the study, said in a statement issued by NASA's Jet Propulsion Laboratory.

KELT-9b is what's known as an "ultrahot Jupiter"—exoplanets which are similar in size to Jupiter in our own solar system, but orbit extremely close to their host star. As a result, they experience incredibly high temperatures to the point where some display physical processes which are more similar to a star than a planet.

In the case of KELT-9b, which is 30 times closer to its star than Earth is to the sun, surface temperatures can reach up to 7,800 degrees Fahrenheit.

The team observed KELT-9b with NASA's Spitzer Space Telescope, which is designed to detect infrared radiation.

The observatory, which was launched in 2003, contains highly sensitive instruments enabling astronomers to study regions of the universe that optical telescopes cannot see. These include the centers of galaxies and planetary systems that are in the process of forming.

It can also be used to observe relatively cool objects, such as failed stars and planets beyond the solar system.

Artist's illustration of the exoplanet KELT-9b next to its star. NASA/JPL-Caltech

With the help of Spitzer, the astronomers measured variations in the heat being emitted from KELT-9b. Despite the fact that the dayside and night sides always face the same way, the researchers noticed there was not a huge difference in temperature between the two. This indicates that heat is being transferred from one side to the other.

Computer models indicate the most likely explanation for the results was the process of hydrogen molecules being ripped apart in the dayside atmosphere before reforming on the night side.

According to a study published in the Astrophysical Journal Letters in 2018, there are only a "handful" of known ultrahot Jupiters. The researchers said that because KELT-9b is the hottest, it is a good candidate to test ideas about heat recirculation.

Discussing the latest study, Mansfield said the process that appears to be taking place on KELT-9b could also be occurring on other ultrahot Jupiters too. "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," she said.

This is not the first time scientists have found extreme processes on KELT-9b.

Last year, a study was published in the the journal Nature that indicated that KELT-9b's atmosphere was so hot that it vaporizes heavy metals, such as iron and titanium.

This study represented the first time that iron had been directly identified in the atmosphere of an exoplanet. Despite being abundant throughout the universe, the metal has a high melting point and thus requires extreme heat to be found as a gas.