The hottest planet we know of to date is nearly thrice the size of Jupiter and even hotter than many stars. Thus, to call this celestial body a “planet” is kind of a stretch.
The exoplanet KELT-9b very closely orbits a star called KELT-9 in the Cygnus constellation. KELT-9b orbits its star so closely that its surface temperature rises up to 4,300ºC. The planet is so hot that it is only about 927ºC cooler than our sun. Meanwhile, the star KELT-9 is more than twice the size of our sun and is nearly twice as hot.
KELT-9b is also likely to be tidally locked to its star. This means one side of the planet is extremely hot while the other side is relatively cooler. Even so, the cooler side of the planet is still hotter than stars like Proxima Centauri, one of our close neighbors. Though KELT-9b is the hottest planet we know of and is hotter than many stars, it will never actually be a star. The source of its heat is external, while the source of heat for stars is internal atomic fusion.
Astronomers in Arizona and northern Cape Town in South Africa discovered the existence of KELT-9b when its shadow passed in front of its star. The observation systems in both locations had Kilodegree Extremely Little Telescopes (KELTs), which are robotic telescopes surveying exoplanets orbiting bright stars. The telescopes were focusing on KELT-9 when astronomers detected KELT-9b's shadow.
According to observations, astronomers were able to surmise the existence of a large exoplanet due to a dimming of starlight that reaches Earth. Astronomers were able to figure out that KELT-9b orbits around its star every one and a half Earth days.
Thus, the planet is quite close to its star. This also means that the heat bearing down on the planet is quite intense. Because of this, the planet's atmosphere has puffed up to a considerable degree. The planet is 2.8 times the size of Jupiter but is only half as dense. It is also so hot that water, carbon dioxide, and methane molecules can't exist there. However, metal atoms are probably able to take the planet's heat—for a time, at least.
Of course, this is a momentous find and the planet is definitely more than interesting. Astronomers are curious as to how the planet formed, and are interested in knowing more about the planet's features.
However, one thing that astronomers are speculating on is the future of KELT-9b. According to calculations, the star blasts the planet with ultraviolet rays that possibly strip 10 million tonnes of material off the planet per second. Astronomers predict that eventually, the host star's intense heat will cause the planet's atmosphere to evaporate. All that will be left is a rocky core, which the expanding star may also eventually engulf.
Obviously, the planet isn't habitable and likely doesn't host alien life. Nonetheless, it's still an awesome find. The astronomers that discovered the hottest planet we know of so far hope that this discovery will encourage other astronomers to look for strange worlds.
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