K2-21

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]

The K2-21 planetary system^[5]

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🜨

K2-21

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
K2-21, EPIC 206011691, TIC 240766850, 2MASS J22411288-1429202[4]
Database references
SIMBAD	data
Exoplanet Archive	data