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Artist's conception of Kepler-42.jpg
Artist's impression of the Kepler-42 (KOI-961) system
Credit: NASA
Observation data
Epoch J2000      Equinox J2000
Constellation Cygnus
Right ascension 19h 28m 52.56s[1]
Declination 44° 37′ 0.96″[1]
Apparent magnitude (V) 16.12[2]
Spectral type M5V[3]
Radial velocity (Rv) −84.48±0.2[3] km/s
Proper motion (μ) RA: 84[1] mas/yr
Dec.: −423[1] mas/yr
Distance 126±20.5[3] ly
(38.7±6.3 pc)
Mass 0.13±0.05[3] M
Radius 0.17±0.04[3] R
Luminosity 2.40x10−3.0±0.3 L
Temperature 3068±174[3] K
Metallicity [Fe/H] =−0.48±0.17[3]
Rotational velocity (v sin i) 2.9±0.4[3] km/s
Other designations
2MASS J19285255+4437096, USNO-B1.0 1346-00336634, KIC 8561063, LSPM J1928+4437

Kepler-42, formerly known as KOI-961, is a red dwarf located in the constellation Cygnus and approximately 126 light years from the Sun. It has three known extrasolar planets, all of which are smaller than Earth in radius,[4] and likely also in mass.[citation needed]


Kepler-42's mass is estimated to be 0.13 times that of the Sun, and to have a radius 0.17 times that of the Sun, that is, just 1.7 times that of the gas giant Jupiter. Due to its small radius and hence surface area, the luminosity of Kepler-42 is only 0.24% of that of the Sun. Its metallicity is one third of the Sun. Kepler-42 has an appreciable proper motion of up to 431±8 mas/yr.[3] Due to its small size and low temperature, the star's habitable zone is much closer to the star than Earth is to the Sun.

Planetary system[edit]

Earth, Mars and the planets of this system compared to Kepler-20e and Kepler-20f, the first terrestrial-sized exoplanets to be discovered outside of the Solar System
The Kepler-42 system as compared to the Jovian system[note 1]

On 10 January 2012, using the Kepler Space Telescope three transiting planets were discovered in orbit around Kepler-42. These planets' radii range from approximately those of Mars to Venus. The Kepler-42 system is only the second known system containing planets of Earth's radius or smaller (the first was the Kepler-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 some hot Jupiters) resemble the moon systems of giant planets such as Jupiter or Saturn more than it does the Solar 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 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 the radial velocity method, the orbital eccentricity remains unknown.

Based on the orbits of the planets and the luminosity and effective temperature of the host star, the equilibrium temperatures of the planets can be calculated. Assuming an extremely high albedo of 0.9 and absence of greenhouse effect, the outer planet Kepler-42 d would have an equilibrium temperature of about 280 K (7 °C),[5] similar to Earth's 255 K (−18 °C).[6] Estimates for the known planets are in the tables below:

Kepler-42c Kepler-42b Kepler-42d Earth
728 K
455 °C
851 °F
524 K
251 °C
483.8 °F
454 K
181 °C
357.8 °F
255 K
−18 °C
−0.4 °F
References:[5][note 2]
The Kepler-42 planetary system[2][4][7]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
c <2.06 M 0.006 0.45328509 ± 9.7x10−7 0 0.73 ± 0.20 R
b <2.73 M 0.0116 1.2137672 ± 4.6x10−6 0 0.78 ± 0.22 R
d <0.9 M 0.0154 1.856169 ± 1.4x10−5 0 0.57 ± 0.18 R


  1. ^ Orbit sizes to scale with each other, but not to the sizes of their respective host bodies
  2. ^ Temperature values for all planets assuming an albedo of 0.3, Earth's value.


  1. ^ a b c d "KOI-961". SIMBAD. Centre de données astronomiques de Strasbourg. 2012-01-11. Retrieved 2012-01-19. 
  2. ^ a b "Star: KOI-961 – 3 PLANETS". Extrasolar Planets Encyclopaedia. 2012-01-12. Retrieved 2012-01-12. 
  3. ^ a b c d e f g h i Philip S. Muirhead; John Asher Johnson; Kevin Apps; Joshua A. Carter; Timothy D. Morton; Daniel C. Fabrycky; J. Sebastian Pineda; Michael Bottom; Barbara Rojas-Ayala; Everett Schlawin; Katherine Hamren; Kevin R. Covey; Justin R. Crepp; Keivan G. Stassun; Joshua Pepper; Leslie Hebb; Evan N. Kirby; Andrew W. Howard; Howard T. Isaacson; Geoffrey W. Marcy; David Levitan; Tanio Diaz-Santos; Lee Armus; James P. Lloyd (2012). "Characterizing the Cool KOIs III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets". arXiv:1201.2189Freely accessible [astro-ph]. 
  4. ^ a b "KOI-961: A Mini-Planetary System". NASA Ames Research Center Kepler, A Search for Habitable Planets. NASA Ames Research Center Kepler. 2012-01-11. Retrieved 2012-01-11. 
  5. ^ a b "Planet Equilibrium Temperature". Habitable Exoplanets Catalog. Planetary Habitability Laboratory at the University of Puerto Rico. Retrieved 29 February 2012. 
  6. ^ "Radiating Equilibrium Temperature". University of Wisconsin Marathon County - Department of Geology/Geography. Retrieved 29 February 2012. 
  7. ^