KELT-9b

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.^[4] Detected using the Kilodegree Extremely Little Telescope, the discovery of KELT-9b was announced in 2016.^[5][1] As of June 2017, it is the hottest known exoplanet.^[6]

KELT-9b

Artist's impression of KELT-9b and its parent star
Orbital characteristics
Semi-major axis	0.03462+0.00110 −0.00093 AU
Orbital period (sidereal)	1.4811235±0.0000011[1] d
Inclination	86.79±0.25[1]
Star	HD 195689
Physical characteristics
Mean radius	1.891+0.061 −0.053[1] RJ
Mass	2.17±0.56[2] MJ
Mean density	0.53000 ± 0.00015 g/cm3[citation needed]
Albedo	<0.14[3]
Temperature	4050±180[1] K

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 10,170 K (9,897 °C; 17,846 °F), 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 (7,157 °C; 12,914 °F); no B-type stars were previously known to host planets. KELT-9, classified as B9.5-A0^[1][7] 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.^[8][9]

Physical properties

Exoplanet KELT-9b orbits host star KELT-9

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.^[9] The outer boundary of its atmosphere nearly reaches its Roche lobe, implying that the planet is experiencing rapid atmospheric escape^[10] driven by the extreme amount of radiation it receives from its host star.^[9][8] In 2020, atmospheric loss rate was measured to be equal to 18 - 68 Earth masses per billion years.^[11]

The planet's elemental abundances remain largely unknown as of 2022, but a low carbon-to-oxygen ratio is strongly suspected.^[12]

This graph shows the average temperature and mass relative to Jupiter (Mj) of known exoplanets as of 2022

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 driven by the latent heat of dissociation and recombination of the molecular hydrogen.^[3] Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in the planetary atmosphere,^[16] although higher resolution spectra taken in 2021 have not found any molecular emissions from the planetary dayside.^[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.^[18]

Size comparison

Jupiter	KELT-9b

See also

• 51 Pegasi b, the first-discovered Hot Jupiter
• List of hottest exoplanets