KELT-21b

KELT-21b is an extrasolar planet discovered in 2017. It is a hot Jupiter with radius of about 1.586 M_J.^[1]

KELT-21b

Discovery
Discovered by	Marshall Johnson et al.[1]
Discovery site	KELT
Discovery date	2018
Detection method	Transit method
Orbital characteristics
Semi-major axis	0.05224 AU (7,815,000 km)
Orbital period (sidereal)	3.6127647[1] d
Inclination	86.46[1]
Star	KELT-21 A (HD 332124)
Physical characteristics
Mean radius	1.586[1] RJ
Mass	<3.91[1] MJ
Mean density	<1.24 g cm−3
Temperature	2,051 K (1,778 °C; 3,232 °F)[1]

Host star

KELT-21b orbits KELT-21 A (HD 332124). It orbits the primary star in a triple star system, with the other two stars located 1.2 arcseconds away. These two stars, designated KELT-21 B and C, have masses of 0.13 M_☉ and 0.11 M_☉, respectively.^[1] The primary star is heavy at 1.458±0.029M_☉, extremely hot at 8210±771 K and rapidly rotating (equatorial velocity equal to 141 km/s).^[2] In comparison, the Sun has a temperature of 5772 K^[3] and rotates at an equatorial velocity of 1.997 km/s.^[4] The planetary orbit is well aligned with the equatorial plane of the host star, with misalignment equal to −5.6^+1.7
_−1.9°.^[5]

Transmission planetary spectroscopy was performed in 2021, based on a single transit observation in 2019. High planetary gravity and relatively low planetary temperature made detection of an atmosphere impossible that time.^[6]