K2-18

K2-18, also known as EPIC 201912552, is a red dwarf star with two planetary companions located 124 light-years (38 parsecs)^[4] from Earth, in the constellation of Leo.

K2-18

Artist's impression of the K2-18 system, with K2-18 on left, K2-18b on right, and K2-18c between. Credit: ESA/Hubble
Approximate two-dimensional location of the star (in red circle); Sigma Leonis is the nearest bright star, which is in a southerly direction, and the boundary of Virgo is similarly far.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Leo
Right ascension	11h 30m 14.51774s[1]
Declination	+07° 35′ 18.2553″[1]
Apparent magnitude (V)	13.50[2]
Characteristics
Evolutionary stage	Red dwarf
Spectral type	M2.8[3]
Astrometry
Radial velocity (Rv)	0.02±0.52[1] km/s
Proper motion (μ)	RA: −80.479 mas/yr[1] Dec.: −133.007 mas/yr[1]
Parallax (π)	26.2469±0.0266 mas[1]
Distance	124.3 ± 0.1 ly (38.10 ± 0.04 pc)
Details
Mass	0.495±0.004[4] M☉
Radius	0.469±0.010[4] R☉
Luminosity	0.0234[5] L☉
Temperature	3,645±52[6] K
Metallicity [Fe/H]	+0.10±0.12[6] dex
Rotation	38.9 to 40.2[6] days
Age	2.9 to 3.1[6] Gyr
Other designations
Gaia DR3 3910747531814692736, EPIC 201912552, 2MASS J11301450+0735180[7]
Database references
SIMBAD	data

Its name is because it was discovered by the K2 Mission, which extended the mission of the Kepler Space Telescope after failure of two of its reaction wheels.

Planetary system

See also: K2-18b

The star has a transiting exoplanet, called K2-18b, a super-Earth located within the habitable zone of K2-18.^[8][9] It was discovered in 2015 by the Kepler space telescope in its K2 mission.^[3] It is the first exoplanet in the habitable zone, albeit a hydrogen-rich sub-Neptune,^[10] to have its atmosphere characterized; initially thought to contain water vapor,^[11] more recent observations have instead detected methane and carbon dioxide.^[12] The presence of these molecules and non-detection of ammonia is consistent with predictions for a hycean planet.^[12]

A second, non-transiting planet, K2-18c, was discovered in 2017 by radial velocity with HARPS.^[13] This planet was challenged by another team with CARMENES data,^[14] but its existence was reaffirmed by the discovery team based on both HARPS and CARMENES data.^[4] This planet has also been confirmed by a later independent study.^[15] System tidal simulation suggests that K2-18c is a gas-rich, Neptune-like planet, similar to K2-18b.^[16]

The K2-18 planetary system^[16]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
c	≥5.62±0.84 M🜨	0.0670 ± 0.0002	9.2072±0.0065[15]	<0.2	—	—
b	8.63±1.35 M🜨	0.1591±0.0004	32.94488±0.00281	—	—	2.711±0.065 R🜨