| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Aquarius[1] |
| Right ascension | 22h22m 29.8611s[2] |
| Declination | −07° 57′ 19.853″[2] |
| Apparent magnitude (V) | 16.06[3] |
| Characteristics | |
| Spectral type | M4V[4][5] |
| Apparent magnitude (J) | 11.695±0.030[6] |
| Apparent magnitude (H) | 11.028±0.023[6] |
| Apparent magnitude (K) | 10.746±0.023[6] |
| Variable type | Planetary transit variable[4] |
| Astrometry | |
| Radial velocity (Rv) | 11.7[5] km/s |
| Proper motion (μ) | RA: −254.604(41)[2] mas/yr Dec.: −194.554(30)[2] mas/yr |
| Parallax (π) | 15.8734 ± 0.0343 mas[2] |
| Distance | 205.5 ± 0.4 ly (63.0 ± 0.1 pc) |
| Details[4] | |
| Mass | 0.257±0.048 M☉ |
| Radius | 0.288±0.028 R☉ |
| Surface gravity (log g) | 4.93±0.04 cgs |
| Temperature | 3214±60 K |
| Metallicity [Fe/H] | 0.26±0.10 dex |
| Other designations | |
Gaia DR2 2622296783699476864, LP 700-6, NLTT 53655, EPIC 206318379[7] | |
| Database references | |
| SIMBAD | data |
K2-28 is a metal rich M4-type main sequence star. One confirmed transiting exoplanet is known to orbit this star. There is another star 5.2 arcseconds to the north–east of K2-28. However, this star has a different proper motion, and is therefore physically unrelated and probably a background star.[4]
K2-28b was first noticed as a candidate extrasolar planet by Vanderburg et al. in 2016, who, in a search of 59,174 stars from the Kepler space telescope's first year of K2 observations, found 234 planetary candidates.[8] Shortly thereafter the K2-ESPRINT Project confirmed that the candidate was a super-Earth sized planet in a close orbit around a red dwarf star.[4]
K2-28b is a sub-Neptune sized planet orbiting its star in only 2.26 days. Despite its short orbital period the equilibrium temperature of the planet is a relatively low 500 Kelvin due to the low luminosity of the parent star.[4] Because of the very small size of the parent star, this planet is a particularly favorable target for transmission spectroscopy by the James Webb Space Telescope, which should be able to determine if the atmosphere is cloudy or clear by observing roughly 5 transits.[9] Among a group of small and cool planets orbiting relatively bright M-dwarfs, its predicted secondary eclipse depth of 230 parts-per-million is second only to Gliese 1214 b.[3]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| K2-28b | 7.18+5.92 −3.08 (estimate) M🜨 |
0.0191+0.0037 −0.0029 |
2.2604455±0.0000010 | 0 | 87.1+0.90 −0.74° |
2.56+0.27 −0.26 R🜨 |
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