View source for Astronomy:Kepler-42

{{short description|Red dwarf star in the constellation Cygnus}}
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{{Starbox begin
| name = Kepler-42
}}
{{Starbox image
| image = [[File:Artist's conception of Kepler-42.jpg|250px]]
| caption = Artist's impression of the Kepler-42 (KOI-961) system
| credit = NASA
}}
{{Starbox observe
| epoch = J2000
| constell = [[Astronomy:Cygnus (constellation)|Cygnus]]
| ra = {{RA|19|28|52.5688}}<ref name="Gaia DR2"/>
| dec = {{DEC|44|37|08.990}}<ref name="Gaia DR2"/>
| appmag_v = 16.12<ref name="Exoplanet.eu"/>
}}
{{Starbox character
| class = M5V<ref name="Muirhead"/>
| b-v =
| u-b =
| variable =
}}
{{Starbox astrometry
| radial_v = −84.48±0.2<ref name="Muirhead"/>
| prop_mo_ra = {{val|93.126|0.073}}<ref name="Gaia DR2"/>
| prop_mo_dec = {{val|−417.420|0.067}}<ref name="Gaia DR2"/>
| parallax = 24.9340
| p_error = 0.0401
| parallax_footnote = <ref name="Gaia DR2"/>
| absmag_v =
}}
{{Starbox detail
| mass = 0.144{{±|0.007|0.006}}<ref name=Mann2017>{{citation|arxiv=1705.01545|title=THE GOLD STANDARD: ACCURATE STELLAR AND PLANETARY PARAMETERS FOR EIGHT Kepler M DWARF SYSTEMS ENABLED BY PARALLAXES|year=2017|doi=10.3847/1538-3881/aa7140|last1=Mann|first1=Andrew W.|last2=Dupuy|first2=Trent|last3=Muirhead|first3=Philip S.|last4=Johnson|first4=Marshall C.|last5=Liu|first5=Michael C.|last6=Ansdell|first6=Megan|last7=Dalba|first7=Paul A.|last8=Swift|first8=Jonathan J.|last9=Hadden|first9=Sam|journal=The Astronomical Journal|volume=153|issue=6|page=267|bibcode=2017AJ....153..267M|s2cid=119325474 |doi-access=free }}</ref>
| radius = 0.175{{±|0.006}}<ref name=Mann2017/>
| luminosity = 3.08{{±|0.28}}x10<sup>−3.0</sup><ref name=Mann2017/>
| temperature = 3269{{±|19}}<ref name=Mann2017/>
| metal_fe = −0.48±0.17<ref name="Muirhead"/>
| rotation =
| rotational_velocity = 2.9±0.4<ref name="Muirhead"/>
| gravity =
| age_gyr =
}}
{{Starbox catalog
| names = [[Astronomy:2MASS|2MASS]]&nbsp;J19285255+4437096, [[Astronomy:Kepler Input Catalog|KIC]] 8561063, LSPM J1928+4437, Gaia DR2 2126556132093765888, KOI-961
}}
{{Starbox reference
| Simbad = Kepler-42
| KIC = 8561063
}}
{{Starbox end}}

'''Kepler-42''', formerly known as '''KOI-961''', is a [[Astronomy:Red dwarf|red dwarf]] located in the [[Astronomy:Constellation|constellation]] [[Astronomy:Cygnus (constellation)|Cygnus]] and approximately 131 light years from the [[Astronomy:Sun|Sun]]. It has three known extrasolar planets, all of which are smaller than [[Astronomy:Earth|Earth]] in radius,<ref name="Kepler"/> and likely also in mass.{{citation needed|date=October 2012}}

==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 [[Astronomy:Gas giant|gas giant]] [[Astronomy:Jupiter|Jupiter]]. Due to its small radius and hence surface area, the [[Astronomy:Luminosity|luminosity]] of Kepler-42 is only 0.24% of that of the Sun. Its [[Astronomy:Metallicity|metallicity]] is one third of the Sun's. Kepler-42 has an appreciable [[Astronomy:Proper motion|proper motion]] of up to 431±8 mas/yr.<ref name="Muirhead"/> Due to its small size and low temperature, the star's [[Astronomy:Habitable zone|habitable zone]] is much closer to the star than Earth is to the Sun.

==Planetary system==
[[File:Comparing the size of Earth, Mars, and exoplanets of Kepler-20 and Kepler-42.jpg|thumb|300px|left|Earth, Mars and the planets of this system compared to [[Astronomy:Kepler-20e|Kepler-20e]] and Kepler-20f, the first terrestrial-sized exoplanets to be discovered outside of the Solar System]] [[File:Kepler-42 System – Jupiter System Comparison.jpg|thumb|300px|left|The Kepler-42 system as compared to the Jovian system<ref group=note>Orbit sizes to scale with each other, but not to the sizes of their respective host bodies</ref>]]
The planetary system comprising three transiting planets was discovered in February 2011<ref name="Fressin2011">{{cite journal | title=Kepler-10 c: a 2.2 Earth Radius Transiting Planet in a Multiple System | last1=Fressin | first1=François | last2=Torres | first2=Guillermo | last3=Désert | first3=Jean-Michel | last4=Charbonneau | first4=David | last5=Batalha | first5=Natalie M. | last6=Fortney | first6=Jonathan J. | last7=Rowe | first7=Jason F. | last8=Allen | first8=Christopher | last9=Borucki | first9=William J. | last10=Brown | first10=Timothy M. | last11=Bryson | first11=Stephen T. | last12=Ciardi | first12=David R. | last13=Cochran | first13=William D. | last14=Deming | first14=Drake | last15=Dunham | first15=Edward W. | last16=Fabrycky | first16=Daniel C. | last17=Gautier Iii | first17=Thomas N. | last18=Gilliland | first18=Ronald L. | last19=Henze | first19=Christopher E. | last20=Holman | first20=Matthew J. | last21=Howell | first21=Steve B. | last22=Jenkins | first22=Jon M. | last23=Kinemuchi | first23=Karen | last24=Knutson | first24=Heather | last25=Koch | first25=David G. | last26=Latham | first26=David W. | last27=Lissauer | first27=Jack J. | last28=Marcy | first28=Geoffrey W. | last29=Ragozzine | first29=Darin | last30=Sasselov | first30=Dimitar D. | display-authors=1 | journal=The Astrophysical Journal Supplement Series | volume=197 | issue=1 | at=5 | year=2011 | arxiv=1105.4647 | bibcode=2011ApJS..197....5F | doi=10.1088/0067-0049/197/1/5 | s2cid=38317440 }}</ref> and confirmed on 10 January 2012, using the [[Engineering:Kepler (spacecraft)|Kepler Space Telescope]]. These planets' radii range from approximately those of [[Astronomy:Mars|Mars]] to [[Astronomy:Venus|Venus]]. The Kepler-42 [[Astronomy:Planetary system|system]] is only the second known system containing planets of Earth's radius or smaller (the first was the [[Astronomy:Kepler-20|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 [[Astronomy:Jupiter|Jupiter]] or [[Astronomy:Saturn|Saturn]] more than it does the [[Astronomy:Solar System|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 [[Astronomy:Orbital eccentricity|orbital eccentricity]] remains unknown.

Based on the orbits of the planets and the luminosity and effective temperature of the host star, the [[Physics:Thermal equilibrium|equilibrium temperatures]] of the planets can be calculated. Assuming an extremely high [[Physics:Bond albedo|albedo]] of 0.9 and absence of [[Earth:Greenhouse effect|greenhouse effect]], the outer planet Kepler-42 d would have an equilibrium temperature of about {{convert|280|K|°C}},<ref name=hec /> similar to [[Astronomy:Earth|Earth]]'s {{convert|255|K|°C}}.<ref name=u-wisc-earth-eqtemp>{{cite web|title=Radiating Equilibrium Temperature |url=http://www.marathon.uwc.edu/geography/100/rad-temp.htm |publisher=University of Wisconsin Marathon County - Department of Geology/Geography |access-date=29 February 2012 |url-status=dead |archive-url=https://web.archive.org/web/20060903192220/http://www.marathon.uwc.edu/geography/100/rad-temp.htm |archive-date= 3 September 2006 }}</ref> Estimates for the known planets are in the tables below:

{| class="wikitable" style="margin:auto;"
|- style="text-align:center;"
|  style="background:#fbb;"| Temperature<br>comparisons
|  style="background:#fda;"| [[Astronomy:Kepler-42c|Kepler-42c]]
|  style="background:#ffa;"| [[Astronomy:Kepler-42b|Kepler-42b]]
|  style="background:#cceed0;"|[[Astronomy:Kepler-42d|Kepler-42d]]
|  style="background:#ade;"| [[Astronomy:Earth|Earth]]

|-
| style="text-align:center;"|Global<br>equilibrium<br>temperature || style="text-align:center;"| 728 K<br> 455&nbsp;°C <br>851&nbsp;°F ||style="text-align:center;"|  524 K<br> 251&nbsp;°C <br> 483.8&nbsp;°F|| style="text-align:center;"| 454 K<br> 181&nbsp;°C <br> 357.8&nbsp;°F ||style="text-align:center;"|  255 K <br> −18&nbsp;°C <br> −0.4&nbsp;°F
|-
| colspan="5" style="text-align:left;"| References:<ref name=hec>{{cite web|title=Planet Equilibrium Temperature|url=http://phl.upr.edu/projects/habitable-exoplanets-catalog/calculators|work=Habitable Exoplanets Catalog|publisher=Planetary Habitability Laboratory at the University of Puerto Rico|access-date=29 February 2012|archive-date=24 August 2017|archive-url=https://web.archive.org/web/20170824164045/http://phl.upr.edu/projects/habitable-exoplanets-catalog/calculators|url-status=dead}}</ref><ref group=note>Temperature values for all planets assuming an [[Physics:Bond albedo|albedo]] of 0.3, Earth's value.</ref>
|}

{{OrbitboxPlanet begin
| name = Kepler-42
| table_ref= <ref name=Mann2017/><ref>{{cite web | url=http://kepler.nasa.gov/Mission/discoveries/ | archive-url=https://web.archive.org/web/20100527104316/http://kepler.nasa.gov/Mission/discoveries/ | url-status=dead | archive-date=2010-05-27 | title=How many exoplanets has Kepler discovered?| date=2015-04-09}}</ref>
}}
{{OrbitboxPlanet
| exoplanet = [[Astronomy:Kepler-42c|c]]
| mass_earth = <2.06
| radius_earth = 0.73{{±|0.03}}
| period = 0.45328731{{±|0.00000005}}
| semimajor = 0.006
| eccentricity = 0
}}
{{OrbitboxPlanet
| exoplanet = b
| mass_earth = <2.73
| radius_earth = 0.76{{±|0.03}}
| period = 1.21377060{{±|0.00000023|0.00000025}}
| semimajor = 0.0116
| eccentricity = 0
}}
{{OrbitboxPlanet
| exoplanet = d
| mass_earth = <0.9
| radius_earth = 0.67{{±|0.04|0.03}}
| period = 1.86511236{{±|0.00000075|0.00000071}}
| semimajor = 0.0154
| eccentricity = 0
}}
{{Orbitbox end}}

==Notes==
{{reflist|group=note}}

==References==
{{reflist|refs=

<ref name=Exoplanet.eu>{{cite web | date=2012-01-12 | url=http://exoplanet.eu/star.php?st=KOI-961 | archive-url=https://web.archive.org/web/20120208054043/http://exoplanet.eu/star.php?st=KOI-961 | url-status=dead | archive-date=February 8, 2012 | work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] | title=Star: KOI-961 – 3 PLANETS | access-date=2012-01-12}}</ref>

<ref name="Gaia DR2">{{Cite Gaia DR2|2126556132093765888}}</ref>

<ref name=Kepler>{{cite web | date=2012-01-11 | url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=179 | archive-url=https://web.archive.org/web/20120314181047/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=179 | url-status=dead | archive-date=2012-03-14 | title=KOI-961: A Mini-Planetary System | work=NASA Ames Research Center Kepler, A Search for Habitable Planets | publisher=NASA Ames Research Center Kepler | access-date=2012-01-11}}</ref>

<ref name="Muirhead">{{cite journal| author1=Philip S. Muirhead | author2=John Asher Johnson | author3=Kevin Apps | author4=Joshua A. Carter | author5=Timothy D. Morton | author6=Daniel C. Fabrycky | author7=J. Sebastian Pineda | author8=Michael Bottom | author9=Barbara Rojas-Ayala | author10=Everett Schlawin | author11=Katherine Hamren | author12=Kevin R. Covey | author13=Justin R. Crepp | author14=Keivan G. Stassun | author15=Joshua Pepper | author16=Leslie Hebb | author17=Evan N. Kirby | author18=Andrew W. Howard | author19=Howard T. Isaacson | author20=Geoffrey W. Marcy | author21=David Levitan | author22=Tanio Diaz-Santos | author23=Lee Armus | author24=James P. Lloyd |year=2012 | title=Characterizing the Cool KOIs III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets | arxiv=1201.2189 | language=en | doi=10.1088/0004-637X/747/2/144 | volume=747 | issue=2 | journal=The Astrophysical Journal | page=144 | bibcode=2012ApJ...747..144M
| s2cid=14889361 }}</ref>

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}}

{{Stars of Cygnus}}
{{2012 in space}}

[[Category:M-type main-sequence stars]]
[[Category:Cygnus (constellation)]]
[[Category:Planetary transit variables]]
[[Category:2MASS objects|J19285255+4437096]]

{{Sourceattribution|Kepler-42}}