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]
Distance272.6 ± 0.3 ly
(83.6 ± 0.1 pc)
Details[3]
Mass0.64±0.11 M☉
Radius0.60±0.10 R☉
Luminosity (bolometric)0.086±0.064 L☉
Temperature4043±375 K
Metallicity [Fe/H]−0.11±0.13 dex
Age>1 Gyr
Other designations
EPIC 206011691, TIC 240766850, 2MASS J22411288-1429202[4]
Database references
SIMBADdata

References
  1. "Finding the constellation which contains given sky coordinates". djm.cc. 2 August 2008.
  2. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  3. Petigura, Erik A.; et al. (2015). "Two Transiting Earth-Size Planets Near Resonance Orbiting a Nearby Cool Star". The Astrophysical Journal. 811 (2): 102. arXiv:1507.08256. Bibcode:2015ApJ...811..102P. doi:10.1088/0004-637X/811/2/102.
  4. "K2-21". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 24 May 2023.
  5. El Moutamid, Maryame; Stevenson, Kevin B.; et al. (April 2023). "Mass derivation of planets K2-21b and K2-21c from transit timing variations". Monthly Notices of the Royal Astronomical Society. 520 (3): 4226–4234. arXiv:2305.11359. Bibcode:2023MNRAS.520.4226E. doi:10.1093/mnras/stad238.


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