K2-28

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]

K2-28

Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius[1]
Right ascension	22h 22m 29.8611s[2]
Declination	−07° 57′ 19.853″[2]
Apparent magnitude (V)	16.06[3]
Characteristics
Evolutionary stage	main sequence[3]
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☉
Luminosity	0.0118[5] L☉
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

Planetary system

Discovery

K2-28 transit light curve from the Spitzer Space Telescope.^[3]

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]

Characteristics

Secondary eclipse depth vs. temperature of small and cool planets orbiting relatively bright M-dwarfs^[3]

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]

The K2-28 planetary system^[9]

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🜨