KELT-9b


KELT-9b is an exoplanet—more specifically, an 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, KELT-9b was announced in 2016.

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 and singly ionized titanium, only to temporarily reform once they reach the cooler night side.