KELT-9b is an exoplanet and ultra-hot Jupiter that orbits the late B-type/early A-type star KELT-9,[4] located about 670 light-years from Earth.

[11] The planet's elemental abundances remain largely unknown as of 2022, but a low carbon-to-oxygen ratio is strongly suspected.

[12] As of 2022[update], KELT-9b is the hottest known exoplanet, with dayside temperatures approaching 4,600 K (4,327 °C; 7,820 °F) — warmer than some K-type stars.

[1][4] Molecules on the day side are broken into their component atoms, so that normally sequestered refractory elements can exist as atomic species, including neutral oxygen,[13] neutral and singly ionized atomic iron[14] (Fe and Fe+) and singly ionized titanium (Ti+),[15] only to temporarily reform once they reach the cooler night side,[4] which is indirectly confirmed by measured enhanced heat transfer efficiency of 0.3 between dayside and nightside, likely diven by the latent heat of dissociation and recombination of the molecular hydrogen.

[17] The thermosphere layer of KELT-9b is expected to heat up to 10,000–11,000 K (9,727–10,727 °C; 17,540–19,340 °F), driven by ionization of heavy metals atoms like iron.