At 4,600 Kelvin, the exoplanet KELT-9b is hotter than most stars in the Milky Way. The heat is so extreme that heavy metals like iron and titanium float freely as atmospheric gas.
Located roughly 670 light-years away in the constellation Cygnus, this "ultra-hot Jupiter" is a gas giant nearly three times the mass of Jupiter, representing a class of planetary bodies pushed to their absolute physical limits.
KELT-9b's extreme temperature is the direct result of its orbit. It sits agonizingly close to its host star, a massive blue A-type star called KELT-9, completing a full orbit in just 1.5 Earth days. Because of this proximity, the planet is tidally locked, meaning the same hemisphere faces the star at all times.
The conditions on the day side of KELT-9b are so violent that basic chemistry breaks down. In a normal gas giant's atmosphere, elements like hydrogen bond together to form molecules. On the day side of KELT-9b, the stellar radiation is so intense that molecular hydrogen is ripped apart into individual atoms. These isolated atoms are then carried by fierce winds to the relatively cooler night side of the planet, where they briefly recombine into molecules before being dragged back to the day side and torn apart again.
The intense bombardment of ultraviolet radiation from the host star is actively destroying KELT-9b. The star's energy is boiling the planet's atmosphere away into space at a rate of over 100,000 tons per second. This rapid evaporation has formed an enormous, glowing tail of escaping gas trailing behind the planet like a comet. Eventually, KELT-9b will either be entirely stripped of its gaseous envelope, leaving behind a barren core, or it will be consumed entirely as its host star ages and expands.