The K2-21 system contains 2 exoplanets. It is located 272.81 light years away from the solar system.
| Mass | 0.68 solar masses |
| Radius | 0.65 solar radiae |
| Temperature | 4222.06 kelvin |
| Stellar Metallicity | -0.11 decimal exponent |
| Age | 1 billion years |
With a mass below 5 Earth masses, a density of 2.7, and a semi-major axis of 0.076 astronomical units, K2-21 b could, potentially, be an ocean world - a planet with no dry land.
| Mass | 4.040 Earth masses |
| Density | 2.700 grams per cubic centimeter |
| Radius | 1.840 Earth radiae |
| Semi-major Axis | 0.076 AU |
| Eccentricity | 0.1 |
| Orbital Period | 9.325 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2015-10 |
| Reference | Petigura et al. 2015 |
At more than 10 Earth masses, K2-21 c is an ice giant, a planet that is made up mostly of volatiles like water, amonia and methane, and enveloped by a dense hydrogen and helium atmosphere, much like Uranus and Neptune.
| Mass | 6.760 Earth masses |
| Density | 5.400 grams per cubic centimeter |
| Radius | 2.490 Earth radiae |
| Semi-major Axis | 0.107 AU |
| Eccentricity | 0.21 |
| Orbital Period | 15.502 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2015-10 |
| Reference | Petigura et al. 2015 |