The HAT-P-21 system contains 1 exoplanet. It is located 904.36 light years away from the solar system.
| Mass | 1.24 solar masses |
| Radius | 1.21 solar radiae |
| Temperature | 5588 kelvin |
| Stellar Metallicity | 0.01 decimal exponent |
| Age | 10.2 billion years |
At more than 50 Earth masses, HAT-P-21 b is a gas giant, a planet whose mass is mostly made up of hydrogen and helium, like Jupiter and Saturn.
| Mass | 1547.832 Earth masses |
| Density | 4.600 grams per cubic centimeter |
| Radius | 12.442 Earth radiae |
| Semi-major Axis | 0.0494 AU |
| Eccentricity | 0.23 |
| Orbital Period | 4.124 days |
| Discovery Method | Transit |
| Discovery Facility | HATNet |
| Discovery Telescope | Canon 200mm f/1.8L |
| Discovery Instrument | 2K CCD Sensor |
| Discovery Date | 2011-12 |
| Reference | Bakos et al. 2010 |