KELT-9b holds the record for the highest known exoplanet surface—or dayside—temperature. Its permanently illuminated hemisphere reaches about 4,600 K, making it hotter than some K-type stars.
KELT-9b is an ultra-hot Jupiter orbiting the blue-white star KELT-9, roughly 670 light-years away. The planet completes one orbit in only 1.48 days at a distance of about 0.035 astronomical units. It was discovered in 2016 by the Kilodegree Extremely Little Telescope survey.
The heat is so intense that molecules on the dayside can break apart into atoms. Astronomers have detected atomic iron and other heavy elements in its atmosphere, while the planet is also losing atmosphere under the star's extreme radiation. KELT-9b is tidally locked, so one hemisphere always faces its star.
A common mix-up is WASP-33b, another exceptionally hot exoplanet. However, KELT-9b remains the standard answer for the hottest confirmed irradiated exoplanet; “surface temperature” here means the temperature of its observable atmosphere, since it has no solid surface.