KELT-21b is an extrasolar planet discovered in 2017. It is a hot Jupiter with radius of about . 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 and , respectively. The primary star is heavy at 1.458, extremely hot at 8210 K and rapidly rotating (equatorial velocity equal to 141 km/s). In comparison, the Sun has a temperature of 5772 K and rotates at an equatorial velocity of 1.997 km/s. The planetary orbit is well aligned with the equatorial plane of the host star, with misalignment equal to −5.6°.
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.
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