| KELT-21b | |
| Discoverer: | Marshall Johnson et al. |
| Discovery Site: | KELT |
| Discovered: | 2018 |
| Apsis: | astron |
| Semimajor: | 0.05224AU |
| Period: | 3.6127647 d |
| Inclination: | 86.46 |
| Star: | KELT-21 A (HD 332124) |
| Mean Radius: | 1.586 |
| Mass: | <3.91 |
| Density: | <1.24 g cm−3 |
| Single Temperature: | 2051K |
KELT-21b is an extrasolar planet discovered in 2017. It is a hot Jupiter with radius of about .
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[1] and rotates at an equatorial velocity of 1.997 km/s.[2] 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.