K2-21, also known as EPIC 206011691, is a red dwarf star located 273 light-years (84 parsecs) away in the constellation Aquarius. It hosts two known exoplanets, discovered in 2015 by the transit method as part of Kepler's K2 mission.[3] Both planets have significantly lower densities than Earth, indicating that they are not rocky planets and are better described as mini-Neptunes. The inner planet, K2-21b, is less dense than the outer planet, K2-21c.[5]
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Aquarius[1] |
| Right ascension | 22h 41m 12.88625s[2] |
| Declination | −14° 29′ 20.3492″[2] |
| Apparent magnitude (V) | 12.85[3] |
| Characteristics | |
| Evolutionary stage | Main sequence |
| Spectral type | M0.0±0.5V[3] |
| Apparent magnitude (B) | 14.14±0.06[3] |
| Apparent magnitude (V) | 12.85±0.02[3] |
| Apparent magnitude (G) | 12.268±0.003[2] |
| Apparent magnitude (J) | 10.251±0.021[4] |
| Apparent magnitude (H) | 9.633±0.022[4] |
| Apparent magnitude (K) | 9.417±0.020[4] |
| Astrometry | |
| Radial velocity (Rv) | 3.54±0.82[2] km/s |
| Proper motion (μ) | RA: 20.672 mas/yr[2] Dec.: -78.914 mas/yr[2] |
| Parallax (π) | 11.9662 ± 0.0150 mas[2] |
| Distance | 272.6 ± 0.3 ly (83.6 ± 0.1 pc) |
| Details[3] | |
| Mass | 0.64±0.11 M☉ |
| Radius | 0.60±0.10 R☉ |
| Luminosity (bolometric) | 0.086±0.064 L☉ |
| Temperature | 4043±375 K |
| Metallicity [Fe/H] | −0.11±0.13 dex |
| Age | >1 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | 1.59+0.52 −0.44 M🜨 | 0.0731+0.0057 −0.0067[3] | 9.3238+0.0002 −0.0001 | — | 88.54+0.49 −0.59° | 1.93±0.07 R🜨 |
| c | 3.88+1.22 −1.07 M🜨 | 0.1026+0.0079 −0.0094[3] | 15.5017±0.0002 | — | 89.02+0.33 −0.41° | 2.25±0.05 R🜨 |