Kepler-42

Artist's impression of the Kepler-42 (KOI-961) system Credit:NASA
Observation data Epoch J2000      Equinox J2000
Constellation	Cygnus
Right ascension	19h 28m 52.5689s[1]
Declination	44° 37′ 08.990″[1]
Apparent magnitude (V)	16.12[2]
Characteristics
Spectral type	M5V[3]
Astrometry
Radial velocity (Rv)	−84.48±0.2[3] km/s
Proper motion (μ)	RA: 93.074(24)mas/yr[1] Dec.: −417.393(25)mas/yr[1]
Parallax (π)	24.9338±0.0204 mas[1]
Distance	130.8 ± 0.1 ly (40.11 ± 0.03 pc)
Details
Mass	0.144+0.007 −0.006[4] M☉
Radius	0.175±0.006[4] R☉
Luminosity	3.08±0.28x10−3.0[4] L☉
Temperature	3269±19[4] K
Metallicity [Fe/H]	−0.48±0.17[3] dex
Rotational velocity (v sin i)	2.9±0.4[3] km/s
Other designations
2MASS J19285255+4437096,KIC 8561063, LSPM J1928+4437, Gaia DR2 2126556132093765888, KOI-961
Database references
SIMBAD	data
KIC	data

Kepler-42, formerly known asKOI-961, is ared dwarf located in theconstellationCygnus and approximately 131light years from theSun. It has three knownextrasolar planets, all of which are smaller thanEarth in radius and orbit very close to the star.^[5]

Kepler-42's mass is estimated to be 0.13 times that of the Sun, and a radius 0.17 times that of the Sun, just 1.7 times that of thegas giantJupiter. Due to its small radius and hence surface area, theluminosity of Kepler-42 is only 0.24% of that of the Sun. Itsmetallicity is one third of the Sun's. Kepler-42 has an appreciableproper motion of up to 431±8mas/yr.^[3] Due to its small size and low temperature, the star'shabitable zone is much closer to the star than Earth is to the Sun.

The planetary system comprising threetransiting planets was discovered in February 2011^[6] and confirmed on 10 January 2012, using theKepler Space Telescope. These planets' radii range from approximately those ofMars toVenus. The Kepler-42system is only the second known system containing planets of Earth's radius or smaller (the first was theKepler-20 system pictured at left). These planets' orbits are also compact, making the system (whose host star itself has a radius comparable to those of somehot Jupiters) resemble the moon systems of giant planets such asJupiter orSaturn more than it does theSolar System. Despite these planets' small size and the star's being one of the faintest stars in Kepler field with confirmed planets, the detection of these planets was possible due to the small size of the star, causing these planets to block a larger proportion of starlight during their transits.

Not all of the orbital parameters of the system are known. For example, as with all transiting planets that have not had their properties established by means of other methods such as theradial velocity method, theorbital eccentricity remains unknown.

Based on the orbits of the planets and the luminosity and effective temperature of the host star, theequilibrium temperatures of the planets can be calculated. Assuming an extremely highalbedo of 0.9 and absence ofgreenhouse effect, the outer planetKepler-42 d would have an equilibrium temperature of about 280 K (7 °C),^[7] similar toEarth's 255 K (−18 °C).^[8] Estimates for the known planets are in the tables below:

• M-type main-sequence stars
• Cygnus (constellation)
• Planetary systems with three confirmed planets
• Planetary transit variables
• Kepler objects of interest
• 2MASS objects
• TIC objects

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• Short description is different from Wikidata