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]
^Garai, Z.; Pribulla, T.; Kovács, J.; Szabó, Gy M.; Claret, A.; Komžík, R.; Kundra, E. (2022), "Rapidly rotating stars and their transiting planets: KELT-17b, KELT-19Ab, and KELT-21b in the CHEOPS and TESS era", Monthly Notices of the Royal Astronomical Society, 513 (2): 2822–2840, arXiv:2204.09077, doi:10.1093/mnras/stac1095
