| 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 |
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]
| 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🜨 |
References
- ^ "Finding the constellation which contains given sky coordinates". djm.cc. 2 August 2008.
- ^ a b c d e f 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.
- ^ a b c d e f g h 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.
- ^ a b c d "K2-21". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 24 May 2023.
- ^ a b El Moutamid, Maryame; 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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