Kepler-42

Kepler-42, formerly known as KOI-961, is a red dwarf located in the constellation Cygnus and approximately 131 light years from the Sun. It has three known extrasolar planets, all of which are smaller than Earth in radius.^[5]

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

Characteristics

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 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's. 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

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]}

The planetary system comprising three transiting planets was discovered in February 2011^[6] and confirmed on 10 January 2012, using the Kepler Space Telescope. 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 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 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),^[7] similar to Earth's 255 K (−18 °C).^[8] Estimates for the known planets are in the tables below:

Temperature comparisons	Kepler-42c	Kepler-42b	Kepler-42d	Earth
Global equilibrium temperature	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:[7][note 2]

The Kepler-42 planetary system^[4][9]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
c	<2.06 M🜨	0.006	0.45328731±0.00000005	0	—	0.73±0.03 R🜨
b	<2.73 M🜨	0.0116	1.21377060+0.00000023 −0.00000025	0	—	0.76±0.03 R🜨
d	<0.9 M🜨	0.0154	1.86511236+0.00000075 −0.00000071	0	—	0.67+0.04 −0.03 R🜨

Notes

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.

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. "Star: KOI-961 – 3 PLANETS". Extrasolar Planets Encyclopaedia. 2012-01-12. Archived from the original on February 8, 2012. Retrieved 2012-01-12.
3. 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". The Astrophysical Journal. 747 (2): 144. arXiv:1201.2189. Bibcode:2012ApJ...747..144M. doi:10.1088/0004-637X/747/2/144. S2CID 14889361.
4. Mann, Andrew W.; Dupuy, Trent; Muirhead, Philip S.; Johnson, Marshall C.; Liu, Michael C.; Ansdell, Megan; Dalba, Paul A.; Swift, Jonathan J.; Hadden, Sam (2017), "THE GOLD STANDARD: ACCURATE STELLAR AND PLANETARY PARAMETERS FOR EIGHT Kepler M DWARF SYSTEMS ENABLED BY PARALLAXES", The Astronomical Journal, 153 (6): 267, arXiv:1705.01545, Bibcode:2017AJ....153..267M, doi:10.3847/1538-3881/aa7140, S2CID 119325474
5. "KOI-961: A Mini-Planetary System". NASA Ames Research Center Kepler, A Search for Habitable Planets. NASA Ames Research Center Kepler. 2012-01-11. Archived from the original on 2012-03-14. Retrieved 2012-01-11.
6. Fressin, François; et al. (2011). "Kepler-10 c: a 2.2 Earth Radius Transiting Planet in a Multiple System". The Astrophysical Journal Supplement Series. 197 (1). 5. arXiv:1105.4647. Bibcode:2011ApJS..197....5F. doi:10.1088/0067-0049/197/1/5. S2CID 38317440.
7. "Planet Equilibrium Temperature". Habitable Exoplanets Catalog. Planetary Habitability Laboratory at the University of Puerto Rico. Archived from the original on 24 August 2017. Retrieved 29 February 2012.
8. "Radiating Equilibrium Temperature". University of Wisconsin Marathon County - Department of Geology/Geography. Archived from the original on 3 September 2006. Retrieved 29 February 2012.
9. "How many exoplanets has Kepler discovered?". 2015-04-09. Archived from the original on 2010-05-27.