Kepler-442

Kepler-442 is a K-type main-sequence star approximately 1,206 light years from Earth in the constellation Lyra. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets that may be transiting their stars. On January 6, 2015, along with the notable stars of Kepler-438 and Kepler-440, it was announced that the star has an extrasolar planet (a super-Earth) orbiting within the habitable zone, named Kepler-442b.[2]

Kepler-442
Observation data
Epoch J2000      Equinox J2000
Constellation Lyra
Right ascension 19h 01m 27.9743s[1]
Declination +39° 16′ 48.2236″[1]
Apparent magnitude (V) 14.976[2]
Characteristics
Evolutionary stage Main sequence
Spectral type K5V[3][4]
Astrometry
Proper motion (μ) RA: 7.785±0.037[1] mas/yr
Dec.: 1.917±0.038[1] mas/yr
Parallax (π)2.7037 ± 0.0203 mas[1]
Distance1,206 ± 9 ly
(370 ± 3 pc)
Absolute magnitude (MV)7.73+0.28
−0.25[2]
Details
Mass0.61 ± 0.03[2] M☉
Radius0.60 ± 0.02[2] R☉
Luminosity (bolometric)0.117[5] L☉
Luminosity (visual, LV)0.069[nb 1] L☉
Temperature4402 ± 100[2] K
Metallicity [Fe/H]−0.37 ± 0.10[2] dex
Age2.9+8.1
−0.2[2] Gyr
Other designations
Gaia DR2 2100258047339711488, KOI-4742, KIC 4138008, 2MASS J19012797+3916482[6]
Database references
SIMBADdata
Extrasolar Planets
Encyclopaedia		data

Nomenclature and history
The Kepler Space Telescope search volume, in the context of the Milky Way Galaxy.

Prior to Kepler observation, Kepler-442 had the 2MASS catalogue number 2MASS J19012797+3916482. In the Kepler Input Catalog it has the designation of KIC 4138008, and when it was found to have transiting planet candidates it was given the Kepler object of interest number of KOI-4742.

Planetary candidates were detected around the star by NASA's Kepler Mission, a mission tasked with discovering planets in transit around their stars. The transit method that Kepler uses involves detecting dips in brightness in stars. These dips in brightness can be interpreted as planets whose orbits pass in front of their stars from the perspective of Earth, although other phenomenon can also be responsible which is why the term planetary candidate is used.[7]

Following the acceptance of the discovery paper, the Kepler team provided an additional moniker for the system of "Kepler-442".[8] The discoverers referred to the star as Kepler-442, which is the normal procedure for naming the exoplanets discovered by the spacecraft.[2] Hence, this is the name used by the public to refer to the star and its planet.

Candidate planets that are associated with stars studied by the Kepler Mission are assigned the designations ".01" etc. after the star's name, in the order of discovery.[9] If planet candidates are detected simultaneously, then the ordering follows the order of orbital periods from shortest to longest.[9] Following these rules, there was only candidate planet were detected, with an orbital period of 112.3053 days.

The designation b derive from the order of discovery. The designation of b is given to the first planet orbiting a given star, followed by the other lowercase letters of the alphabet.[10] In the case of Kepler-442, there was only one planet detected, so only the letter b is used. The name Kepler-442 derives directly from the fact that the star is the catalogued 442nd star discovered by Kepler to have confirmed planets.

Stellar characteristics

Kepler-442 is a K-type main sequence star that is approximately 61% the mass of and 60% the radius of the Sun. It has a temperature of 4402 K and is about 2.9 billion years old, but the margin of error here is quite large.[2] In comparison, the Sun is about 4.6 billion years old[11] and has a temperature of 5778 K.[12]

The star is somewhat poor in metals, with a metallicity ([Fe/H]) of about –0.37, or about 43% of the amount of iron and other heavier metals found in the Sun.[2] The star's luminosity is a bit low for a star like Kepler-442, with a luminosity of around 12% of that of the solar luminosity.[5]

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.976. Therefore, it is too dim to be seen with the naked eye.

Planetary system
The Kepler-442 planetary system[2]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b 2.3+5.9
−1.3 M🜨		 0.409+0.209
−0.060		 112.3053+0.0024
−0.0028		 0.04+0.08
−0.04		 89.94+0.06
−0.12°		 1.34+0.11
−0.18 R🜨

The only known planet transits the star; this means that the planet's orbit appear to cross in front of their star as viewed from the Earth's perspective. Its inclination relative to Earth's line of sight, or how far above or below the plane of sight it is, vary by less than one degree. This allows direct measurements of the planet's periods and relative diameters (compared to the host star) by monitoring the planet's transit of the star.

Kepler-442b is a super-Earth with a radius 1.34 times that of Earth, and orbits well within the habitable zone. It is likely a rocky planet due to its radius. According to NASA, it was described as being one of the most Earth-like planets, in terms of size and temperature, yet found.[13][14] It is just outside of the zone (around 0.362 AU) where tidal forces from its host star would be enough to tidally lock it.[15]

See also

Notes
  1. With the absolute visual magnitude of Kepler-442, , and the Sun, , the visual luminosity of Kepler-442 is calculated from:

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Torres, Guillermo; Kipping, David M.; Fressin, Francois; Caldwell, Douglas A.; Twicken, Joseph D.; Ballard, Sarah; Batalha, Natalie M.; Bryson, Stephen T.; Ciardi, David R.; Henze, Christopher E.; Howell, Steve B.; Isaacson, Howard T.; Jenkins, Jon M.; Muirhead, Philip S.; Newton, Elisabeth R.; Petigura, Erik A.; Barclay, Thomas; Borucki, William J.; Crepp, Justin R.; Everett, Mark E.; Horch, Elliott P.; Howard, Andrew W.; Kolbl, Rea; Marcy, Geoffrey W.; McCauliff, Sean; Quintana, Elisa V. (2015). "Validation of 12 Small Kepler Transiting Planets in the Habitable Zone". The Astrophysical Journal. 800 (2): 99. arXiv:1501.01101. Bibcode:2015ApJ...800...99T. doi:10.1088/0004-637X/800/2/99. S2CID 8512655.
  3. Fraser Cain (10 January 2020). "The Perfect Stars to Search for Life On Their Planets". Universe Today. Retrieved 23 June 2021.
  4. "Goldilocks Stars Are Best Places to Look for Life". NASA. 21 September 2020 [14 January 2020]. Retrieved 23 June 2021. "Kepler-442 is noteworthy in that this star (spectral classification, K5) hosts what is considered one of the best Goldilocks planets, Kepler-442b, a rocky planet that is a little more than twice Earth's mass. So the Kepler-442 system is a Goldilocks planet hosted by a Goldilocks star!" said Guinan.
  5. Kepler 442b hpcf.upr.edu
  6. "Kepler-442". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-08-21.
  7. Morton, Timothy; Johnson, John (23 August 2011). "On the Low False Positive Probabilities of Kepler Planet Candidates". The Astrophysical Journal. 738 (2): 170. arXiv:1101.5630. Bibcode:2011ApJ...738..170M. doi:10.1088/0004-637X/738/2/170. S2CID 35223956.
  8. NASA (27 January 2014). "Kepler – Discoveries – Summary Table". NASA. Archived from the original on 27 May 2010. Retrieved 1 March 2014.
  9. "Kepler Input Catalog search result". Space Telescope Science Institute. Retrieved 18 April 2013.
  10. Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  11. Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Retrieved 19 February 2011.
  12. Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
  13. Clavin, Whitney; Chou, Felicia; Johnson, Michele (6 January 2015). "NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones". NASA. Retrieved 6 January 2015.
  14. Sample, Ian (7 January 2015). "Kepler 438b: Most Earth-like planet ever discovered could be home for alien life". The Guardian. Retrieved 7 January 2015.
  15. "Planetary Habitability Laboratory at University of Puerto Rico". Archived from the original on 2012-01-08. Retrieved 2016-07-30.
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