View source for K2-239

{{short description|Star}}
{{Starbox begin
| name = K2-239
}}
{{Starbox observe
| epoch = J2000
| constell = [[Sextans (constellation)|Sextans]]
| ra = {{RA|10|42|22.6343}}<ref name="Gaia DR3"/>
| dec = {{DEC|+04|26|28.886}}<ref name="Gaia DR3"/>
| appmag_v = 14.549<ref name="arxiv"/>
}}
{{Starbox character
| type = [[Main sequence]]
| class = M3V<ref name="arxiv"/>
| b-v = 
}}
{{Starbox astrometry
| radial_v = {{val|−21.15|(99)}}<ref name="Gaia DR3"/>
| prop_mo_ra = {{val|−42.338|(19)}}
| prop_mo_dec = {{val|4.147|(22)}}
| pm_footnote = <ref name="Gaia DR3"/>
| parallax = 32.1290
| p_error = 0.0190
| parallax_footnote = <ref name="Gaia DR3"/>
| absmag_v =
}}
{{Starbox detail
| mass = 0.40 ± 0.01<ref name="arxiv"/>
| radius = 0.36 ± 0.01<ref name="arxiv"/>
| rotation = 
| rotational_velocity = 
| luminosity = 0.016 ± 0.001<ref name="arxiv"/>
| temperature = 3420 ± 18<ref name="arxiv"/>
| gravity = 4.9 ± 0.1<ref name="arxiv"/>
| metal_fe = -0.1 ± 0.1<ref name="arxiv"/>
| age_gyr = 
}}
{{Starbox catalog
| names = {{odlist| EPIC=248545986}}
}}
{{Starbox reference
| Simbad = K2-239
}}
{{Starbox end}}

'''K2-239''' (also designated '''EPIC 248545986''') is a small red dwarf star in the constellation [[Sextans (constellation)|Sextans]], about 32 parsecs (101 light-years) away from [[Earth]].<ref name="Gaia DR3"/> Observed by the [[Kepler (spacecraft)|Kepler Space Telescope]] during Campaign 14 of its K2 "Second Light" mission, it was found to have three hot, likely [[Terrestrial planet|rocky]] Earth-sized planets in orbit around it.<ref name="arxiv"/><ref name="bgr"/>

==Stellar characteristics==
K2-239 is a small red dwarf star of spectral class M3V. It is 40% the mass and 36% the radius of the [[Sun]] with just 0.016 times the luminosity. It has a temperature of 3420 K and its age is unknown.<ref name="arxiv"/> For comparison, the Sun has a temperature of 5778 K and is 4.5 billion years old. K2-239 has a visual magnitude of 14.549, far too dim to see with the unaided eye.<ref name="arxiv"/> It is also one of the closer systems found by Kepler, only about 101 light-years away from Earth.<ref name="arxiv"/><ref name="Gaia DR3"/>

==Planetary System==
{{OrbitboxPlanet begin
| name = K2-239
| table_ref= <ref name="arxiv"/>
}}
{{OrbitboxPlanet
| exoplanet = b
| mass_earth = 1.4 ± 0.4
| period = 5.240 ± 0.001
| semimajor = 0.0441 ± 0.0008
| radius_earth = 1.1 ± 0.1
| eccentricity  = 
| inclination = 88.99 {{±|0.68|0.87}}
}}
{{OrbitboxPlanet
| exoplanet = c
| mass_earth = 0.9 ± 0.3
| period = 7.775 ± 0.001
| semimajor = 0.0576 ± 0.0009
| radius_earth = 1.0 ± 0.1
| eccentricity  = 
| inclination = 88.77 {{±|0.70|0.57}}
}}
{{OrbitboxPlanet
| exoplanet = d
| mass_earth = 1.3 ± 0.4
| period = 10.115 ± 0.001
| semimajor = 0.0685 ± 0.0012
| radius_earth = 1.1 ± 0.1
| eccentricity  = 
| inclination = 89.43 {{±|0.38|0.45}}
}}
{{Orbitbox end}}

K2-239 has a system of three small, Earth-sized planets in a tight 2:3:4 orbital resonance chain. All of them are between 1.0 and 1.1 times the size of Earth, meaning they are very likely to be rocky. The discovery team estimated their masses to range from 0.9 {{Earth_mass}} to 1.4 {{Earth_mass}}, consistent with a rocky composition for each of the planets. Due to their proximity to K2-239 they are all hot; however, because the host star is just 1.6% as luminous as the Sun, they are much cooler than if they were placed around the Sun. For albedoes of 0 they would have equilibrium temperatures of {{convert|502|K|C F}} for K2-239b, {{convert|427|K|C F}} for K2-239c, and {{convert|399|K|C F}} for K2-239d. None of them are cool enough to host liquid water or be considered potentially habitable.

[[File:K2-239 System.jpg|thumb|Size comparison of the three known planets of K2-239 (''artistic concept'') with Earth]]

==See also==
*[[K2-148b]]

==References==
{{reflist|refs=

<ref name="Gaia DR3">{{Cite Gaia DR3|3857872051994269824}}</ref>

<ref name="arxiv">{{cite journal|arxiv=1806.01181|last1=Diez Alonso|first1=E|title=Two planetary systems with transiting Earth-size and super-Earth planets orbiting late-type dwarf stars|last2=Gonzalez Hernandez|first2=J. I|last3=Suarez Gomez|first3=S. L|last4=Aguado|first4=D. S|last5=Gonzalez Gutierrez|first5=C|last6=Suarez Mascareno|first6=A|last7=Cabrera-Lavers|first7=A|last8=Gonzalez-Nuevo|first8=J|last9=Toledo Padron|first9=B|last10=Gracia|first10=J|last11=de Cos Juez|first11=F. J|last12=Rebolo|first12=R|journal=Monthly Notices of the Royal Astronomical Society: Letters|year=2018|volume=480|issue=1|pages=L1|doi=10.1093/mnrasl/sly102|doi-access=free |bibcode=2018MNRAS.480L...1D|s2cid=102334726}}</ref>

<ref name="bgr">{{cite web|url=http://bgr.com/2018/06/08/earth-like-planets-exoplanets-k2-239/|author=Mike Wehner|title=Astronomers spot a trio of Earth-sized planets orbiting a distant star|date=8 June 2018 |accessdate=2018-06-19}}</ref>

}}

{{2018 in space}}
{{Stars of Sextans}}

[[Category:M-type main-sequence stars]]
[[Category:Exoplanets discovered by K2]]
[[Category:Sextans]]