KELT-9b

KELT-9b is an exoplanet and ultra-hot Jupiter that orbits the late B-type/early A-type star KELT-9, located about 670 light-years from Earth. Detected using the Kilodegree Extremely Little Telescope, the discovery of KELT-9b was announced in 2016. As of June 2017, it is the hottest known exoplanet.

Host star
The host star, KELT-9, is 2 to 3 times larger and 2 to 3 times more massive than the sun. The surface temperature is 10170 K, unusually hot for a star with a transiting planet. Prior to the discovery of KELT-9b, only six A-type stars were known to have planets, of which the warmest, WASP-33, is significantly cooler at 7430 K; no B-type stars were previously known to host planets. KELT-9, classified as B9.5-A0 could be the first B-type star known to have a planet. KELT-9b occupies a circular but strongly inclined orbit a mere 0.03462 AU from KELT-9 with an orbital period of less than 1.5 days.

Physical properties
KELT-9b is a relatively large giant planet at about 2.8 times the mass of Jupiter; however given that its radius is nearly twice that of Jupiter, its density is less than half that of it. Like many hot Jupiters, KELT-9b is tidally locked with its host star. The outer boundary of its atmosphere nearly reaches its Roche lobe, implying that the planet is experiencing rapid atmospheric escape driven by the extreme amount of radiation it receives from its host star. In 2020, atmospheric loss rate was measured to be equal to 18 - 68 Earth masses per billion years.

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



, KELT-9b is the hottest known exoplanet, with dayside temperatures approaching 4600 K — warmer than some K-type stars. 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, neutral and singly ionized atomic iron (Fe and Fe+) and singly ionized titanium (Ti+), only to temporarily reform once they reach the cooler night side, 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. Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in the planetary atmosphere, although higher resolution spectra taken in 2021 have not found any molecular emissions from the planetary dayside.

The thermosphere layer of KELT-9b is expected to heat up to 10000 - 11000 K, driven by ionization of heavy metals atoms like iron.