K2-21

K2-21
Observation data; Epoch J2000 Equinox J2000
Constellation	Aquarius
Right ascension	22h 41m 12.88625s
Declination	−14° 29′ 20.3492″
Apparent magnitude (V)	12.85
Characteristics
Evolutionary stage	Main sequence
Spectral type	M0.0±0.5V
Apparent magnitude (B)	14.14±0.06
Apparent magnitude (V)	12.85±0.02
Apparent magnitude (G)	12.268±0.003
Apparent magnitude (J)	10.251±0.021
Apparent magnitude (H)	9.633±0.022
Apparent magnitude (K)	9.417±0.020
Astrometry
Radial velocity (Rv)	3.54±0.82 km/s
Proper motion (μ)	RA: 20.672 mas/yr ; Dec.: -78.914 mas/yr
Parallax (π)	11.9662 ± 0.0150 mas
Distance	272.6 ± 0.3 ly ; (83.6 ± 0.1 pc)
Details
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
Database references
SIMBAD	data

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 $R$ _🜨

^ "Finding the constellation which contains given sky coordinates". djm.cc. 2 August 2008.
^ ^a ^b ^c ^d ^e ^f Cite error: The named reference GaiaDR3 was invoked but never defined (see the help page).
^ ^a ^b ^c ^d ^e ^f ^g ^h Cite error: The named reference Petigura2015 was invoked but never defined (see the help page).
^ ^a ^b ^c ^d Cite error: The named reference SIMBAD was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference Moutamid2023 was invoked but never defined (see the help page).

[1] "Finding the constellation which contains given sky coordinates". djm.cc. 2 August 2008.

[GaiaDR3-2] ^ ^a ^b ^c ^d ^e ^f Cite error: The named reference GaiaDR3 was invoked but never defined (see the help page).

[Petigura2015-3] ^ ^a ^b ^c ^d ^e ^f ^g ^h Cite error: The named reference Petigura2015 was invoked but never defined (see the help page).

[SIMBAD-4] Cite error: The named reference SIMBAD was invoked but never defined (see the help page).

[Moutamid2023-5] Cite error: The named reference Moutamid2023 was invoked but never defined (see the help page).

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Observation data Epoch J2000 Equinox J2000
Constellation	Aquarius^[1]
Right ascension	22^h 41^m 12.88625^s^[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 (R_v)	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