Kelt-9b is an extrasolar planet orbits the star Kelt-9. It is a hot Jupiter that is one of the hottest planets known. The planet is also one of the largest planets known, and is located 670 light years away from earth. Kelt-9b is also tidally locked to its host star, meaning that one side always faces its star.
Host star[]
The surface temperature of the host star, KELT-9, is 10,170 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 7,430 K; no B-type stars were previously known to host planets. KELT-9, classified as either B9.5-A0 or A1, 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 Jupiter. 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, atmosphere loss rate was measured to be equal to 18 - 68 Earth masses per billion years.
As of January 2020, KELT-9b is one of the hottest known exoplanets, with dayside temperatures approaching 4,600 K, warmer than many low-mass stars. Molecules on the day side are broken into their component atoms, such that normally sequestered refractory elements can exist as atomic species, including 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. Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in planetary atmosphere.
Thermosphere layer of KELT-9b is expected to heat up to 10000-11000 K, driven by ionization of heavy metals atoms like iron.