Exoplanet’s atmosphere is melting before our eyes

KELT-9b, a hot Jupiter, orbits its sun so closely that a year there lasts just one-and-a-half Earth days. With a surface temperature of 4,300 degrees Celsius, KELT-9b is hotter than any other exoplanet, as well as some stars.

Astronomers discovered this supremely sweltering world orbiting a star some 670 light years from Earth in 2017, and are still learning details about just how unhabitable it is. For example, KELT-9b is so hot that its atmosphere seems to be constantly melting on one side, a new study suggests.

In a recent study published in the astrophysical journal Letters, researchers watched KELT-9b through NASA’s Spitzer space telescope, which observes space in infrared light. This allowed the team to record subtle variations in the planet’s heat as it whizzed around its home star.

Because the planet is tidally locked – meaning the ‘dayside’ always faces the sun while the other side always points away in perpetual night – the team saw remarkable differences in temperature on either side of the planet. Using computer models, the researchers determined that gas and heat were being cycled across the two halves of the globe, resulting in a dramatic circle of atomic destruction and rebirth.

On the dayside, the heat of the sun was so intense that hydrogen molecules in KELT-9b’s atmosphere were literally being ripped to shreds and blown across the planet, a process known as dissociation. While the nightside was still extremely hot at 2,300 degrees Celsius, it appeared to be just cool enough for loose atoms from the dayside to recombine into hydrogen molecules. Eventually, though, those molecules flowed back to the dayside, where they were ripped apart again.