{{Use dmy dates|date=March 2014}} {{Short description|Red dwarf in the constellation Lyra}} {{Starbox begin}} {{Starbox image | image = 250px | caption = Artist's impression of the planets orbiting Kepler-138 }} {{Starbox observe | epoch = J2000 | constell = Lyra<ref name="Roman1987"/> | ra = {{RA|19|21|31.56798}}<ref name="GaiaDR3"/> | dec = {{DEC|+43|17|34.6810}}<ref name="GaiaDR3"/> | appmag_v = {{val|13.040|0.092}}<ref name="NASAExoplanetArchive"/> }} {{Starbox character | type = main sequence<ref name="GaiaDR3"/> | class= M1V<ref name = "Pineda"/> | g-r= 1.121<ref name = "kepler-catalogue"/> | appmag_1_passband = V | appmag_1 = {{val|13.040|0.092}}<ref name="NASAExoplanetArchive"/> | appmag_2_passband = Kepler | appmag_2 = 12.925<ref name="kepler-catalogue"/> }} {{Starbox astrometry | radial_v = {{val|-37.25|0.72}}<ref name="GaiaDR3"/> | prop_mo_ra = {{val|−20.461|0.012}} | prop_mo_dec = {{val|22.641|0.012}} | pm_footnote = <ref name="GaiaDR3"/> | parallax = 14.9019 | p_error = 0.0097 | parallax_footnote = <ref name="GaiaDR3"/> | absmag_v = 8.81 ± 0.28 }} {{Starbox detail | mass = {{val|0.535|0.012}}<ref name="Piaulet2022"/> | radius = {{val|0.535|0.013|0.014}}<ref name="Piaulet2022"/> | luminosity_bolometric = {{val|0.056|0.004}}<ref name="Piaulet2022"/> | luminosity_visual = 0.026 ± 0.006 | temperature = {{val|3726|44|40}}<ref name=Mann2017/> | gravity = {{val|4.71|0.03}}<ref name="Piaulet2022"/> | metal_fe = -0.28 ± 0.10<ref name="Pineda" /> | rotation = {{val|19.394|0.013|s= days}}<ref name="McQuillan2013"/> | rotational_velocity = ~3<ref name="Kipping"/> | age_gyr = >1<ref name="Kipping"/> }} {{Starbox catalog | names = {{odlist | Kepler = 138 | KOI = 314 | KIC = 7603200 | 2MASS = J19213157+4317347 | TIC = 159376971 }}<ref name="SIMBAD"/> }} {{Starbox reference | Simbad = Kepler-138 | NSTED = Kepler-138 | EPE = Kepler-138 | KIC = 7603200 }} {{Starbox end}}
'''Kepler-138''', also known as '''KOI-314''', is a red dwarf<ref name = "Pineda"/><ref name = "Mann"/> located in the constellation Lyra, 219 light years from Earth.<ref name="GaiaDR3"/> It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets transiting their stars.
The star hosts three confirmed planets and a likely fourth, including the lowest-mass exoplanet with a measured mass and size discovered to date, Kepler-138b,<ref name = "Jontof-Hutter"/> with a mass comparable to that of Mars. Kepler-138d is remarkable for its low density; initially thought likely to be a gas dwarf,<ref name = "Kipping"/> more recent observations as of 2022 show that it, as well as planet c, are likely to be ocean worlds.<ref name="NASA2022"/><ref>{{cite web |url=https://arstechnica.com/science/2022/12/scientists-revisit-kepler-findings-learn-two-planets-are-water-worlds/ |title=Scientists may have found the first water worlds |last=Timmer |first=John |date=15 December 2022 |website=Ars Technica |access-date=17 December 2022}}</ref>
==Nomenclature and history== [[File:LombergA1024.jpg|thumb|left|300px|The ''Kepler Space Telescope'' search volume, in the context of the Milky Way Galaxy.]] Prior to Kepler observation, KOI-314 had the 2MASS catalogue number 2MASS J19213157+4317347. In the Kepler Input Catalog it has the designation of KIC 7603200, and when it was found to have transiting planet candidates it was given the Kepler object of interest number of KOI-314.
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 phenomena can also be responsible which is why the term planetary candidate is used.<ref name = "Morton">{{Cite journal | last1=Morton | first1=Timothy | last2=Johnson | first2=John | title=On the Low False Positive Probabilities of Kepler Planet Candidates | journal=The Astrophysical Journal| volume=738 | issue=2 | pages=170 | date=23 August 2011 | doi=10.1088/0004-637X/738/2/170 |arxiv = 1101.5630 |bibcode = 2011ApJ...738..170M | s2cid=35223956 }}</ref> By timing these dips, gravitational interactions were detected between two of the candidates,<ref name = "Kipping"/> allowing for a measurement of their masses and confirmation as real planets given that the masses were significantly below the deuterium burning limits.<ref>{{cite web|date=28 February 2003 |title=Working Group on Extrasolar Planets: Definition of a "Planet" |url=http://www.dtm.ciw.edu/boss/definition.html |work=IAU position statement |access-date=9 September 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060916161707/http://www.dtm.ciw.edu/boss/definition.html |archive-date=16 September 2006 }}</ref>
Following the acceptance of the discovery paper, the Kepler team provided an additional moniker for the system of "Kepler-138".<ref name = "keplernames">{{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=27 May 2010 |title=Kepler – Discoveries – Summary Table |author=NASA | date=27 January 2014 |publisher=NASA |access-date=1 March 2014|author-link=NASA }}</ref> However, the planets were discovered by scientists outside of the Kepler team who referred to the star as KOI-314, as the Kepler designation had not been assigned yet.<ref name = "Kipping"/><ref name = "Briefing">{{cite web |url=https://www.youtube.com/watch?v=9jNcfRsUK8o |title=David Kipping - Press Briefing on First Earth-Mass Transiting Planet, KOI-314c |author=David Kipping | date=26 January 2014 | publisher=YouTube | access-date=1 March 2014}}</ref>
Candidate planets that are associated with stars studied by the Kepler Mission are assigned the designations ".01", ".02", ".03", etc. after the star's name, in the order of discovery.<ref name = "kepler-catalogue"/> If planet candidates are detected simultaneously, then the ordering follows the order of orbital periods from shortest to longest.<ref name = "kepler-catalogue"/> Following these rules, the first two candidate planets were detected simultaneously<ref name = "Borucki">{{Cite journal |last1=Borucki |first1=William J. |author-link=William J. Borucki |title=Characteristics of Planetary Candidates Observed by Kepler. II. Analysis of the First Four Months of Data |journal=The Astrophysical Journal |volume=736 |issue=1 |pages=19 |date=29 June 2011 |doi=10.1088/0004-637X/736/1/19 |arxiv=1102.0541 |bibcode=2011ApJ...736...19B |last2=Koch |first2=David G. |last3=Batalha |first3=Natalie |last4=Brown |first4=Timothy M. |last5=Bryson |first5=Stephen T. |last6=Caldwell |first6=Douglas |last7=Christensen-Dalsgaard |first7=Jørgen |last8=Cochran |first8=William D. |last9=Devore |first9=Edna |last10=Dunham |first10=Edward W. |last11=Gautier |first11=Thomas N. |last12=Geary |first12=John C. |last13=Gilliland |first13=Ronald |last14=Gould |first14=Alan |last15=Howell |first15=Steve B. |last16=Jenkins |first16=Jon M. |last17=Latham |first17=David W. |last18=Lissauer |first18=Jack J. |last19=Marcy |first19=Geoffrey W. |last20=Rowe |first20=Jason |last21=Sasselov |first21=Dimitar |last22=Boss |first22=Alan |last23=Charbonneau |first23=David |last24=Ciardi |first24=David |last25=Doyle |first25=Laurance |last26=Dupree |first26=Andrea K. |last27=Ford |first27=Eric B. |last28=Fortney |first28=Jonathan |last29=Holman |first29=Matthew J. |s2cid=15233153 |display-authors=etal}}</ref> and assigned the names KOI-314.01 and KOI-314.02, with respective orbital periods of 13.8 and 23.1 days.<ref name="Borucki"/> Over a year later, a much smaller planet candidate was detected and assigned the name KOI-314.03,<ref name = "Batalha">{{Cite journal |last1=Batalha |first1=Natalie |title=Planetary Candidates Observed by Kepler. III. Analysis of the First 16 Months of Data |journal=The Astrophysical Journal Supplement |volume=204 |issue=2 |pages=24 |date=5 February 2013 |doi=10.1088/0067-0049/204/2/24 |arxiv=1202.5852 |bibcode=2013ApJS..204...24B |last2=Rowe |first2=Jason F. |last3=Barclay |first3=Thomas |last4=Burke |first4=Christopher J. |last5=Caldwell |first5=Douglas A. |last6=Christiansen |first6=Jessie L. |last7=Mullally |first7=Fergal |last8=Thompson |first8=Susan E. |last9=Brown |first9=Timothy M. |last10=Dupree |first10=Andrea K. |last11=Fabrycky |first11=Daniel C. |last12=Ford |first12=Eric B. |last13=Fortney |first13=Jonathan J. |last14=Gilliland |first14=Ronald L. |last15=Isaacson |first15=Howard |last16=Latham |first16=David W. |last17=Marcy |first17=Geoffrey W. |last18=Quinn |first18=Samuel N. |last19=Ragozzine |first19=Darin |last20=Shporer |first20=Avi |last21=Borucki |first21=William J. |last22=Ciardi |first22=David R. |last23=Gautier |first23=Thomas N. |last24=Haas |first24=Michael R. |last25=Jenkins |first25=Jon M. |last26=Koch |first26=David G. |last27=Lissauer |first27=Jack J. |last28=Rapin |first28=William |last29=Basri |first29=Gibor S. |s2cid=19023502 |display-authors=etal}}</ref> despite being the shortest orbital period planet (period of 10.3 days) found in the system.
Confirmed planets are conventionally assigned the designations ''b'', ''c'', ''d'', etc. after the star's name.<ref name="planetnaming">{{cite arXiv |title=On the naming convention used for multiple star systems and extrasolar planets |last1=Hessman |first1=F. V. |last2=Dhillon |first2=V. S. |last3=Winget |first3=D. E. |last4=Schreiber |first4=M. R. |last5=Horne |first5=K. |last6=Marsh |first6=T. R. |last7=Guenther |first7=E. |last8=Schwope |first8=A. |last9=Heber |first9=U. |eprint=1012.0707 |date=2010 |class=astro-ph.SR }}</ref> The labels are assigned alphabetically in the order of discovery starting from ''b''.<ref name = "planetnaming"/> Since KOI-314.01 and KOI-314.02 were confirmed as planets simultaneously, the alphabetical names were assigned in order of orbital period by the discoverers,<ref name = "Kipping"/> and thus became KOI-314b and KOI-314c respectively. Since no gravitational interactions were detected due to KOI-314.03, this planetary candidate remained unconfirmed as 6 January 2014 and thus kept the same name.<ref name = "Kipping"/>
In the following weeks, on 28 February 2014, a new paper validated KOI-314.03 as being a real planet with a false alarm probability of less than 1%.<ref name="Rowe"/> The new paper used different names for the planets, going from KOI-314b to Kepler-138c, KOI-314c to Kepler-138d and KOI-314.03 to Kepler-138b.<ref name = "Rowe"/> These designations have been used by subsequent studies, and by databases such as the NASA Exoplanet Archive.<ref name="NASAExoplanetArchive"/> This situation is similar to that of some other planetary systems such as Mu Arae, where different designations have been used for the same planets in the literature.
On 16 December 2022, two possible Earth-like water worlds Kepler-138 c and Kepler-138 d were detected in the Kepler 138 system by the Hubble and Spitzer Space Telescopes.
==Stellar characteristics== Kepler-138 is a red dwarf with approximately 54% the mass of and 54% the radius of the Sun.<ref name="Piaulet2022"/> It has a surface temperature of {{val|3726|44|40|ul=K}}.<ref name=Mann2017/> In comparison, the Sun has a surface temperature of 5778 K.<ref>{{cite web |url=http://www.universetoday.com/18092/temperature-of-the-sun/ |title=Temperature of the Sun |author=Fraser Cain |date=15 September 2008 |publisher=Universe Today |access-date=19 February 2011}}</ref> Kepler-138's apparent magnitude (how bright it appears from Earth's perspective) is 13.04,<ref name="NASAExoplanetArchive"/> too dim to be seen with the naked eye.
==Planetary system== The three inner known planets of Kepler-138 transit the star; this means that all three planets' orbits appear to cross in front of their star as viewed from the Earth's perspective. Their inclinations relative to Earth's line of sight, or how far above or below the plane of sight they are, vary by less than one degree. This allows direct measurements of the planets' orbital periods and relative diameters (compared to the host star) by monitoring each planet's transit of the star.<ref name = "Kipping"/><ref name = "Rowe"/> There is also a likely fourth non-transiting planet, Kepler-138e, detected through transit-timing variations.<ref name="Piaulet2022"/><ref name="NASA2022"/>
Although the innermost planet has a size similar to Mars, Kepler-138c and d both have a radius of about 1.5 Earth radii<ref name="Piaulet2022"/> (revised from earlier estimates of 1.2 Earth radii).<ref name = "Kipping"/><ref name = "Rowe"/> Although Kepler-138c and d have similar radii, their masses and densities were initially thought to vary greatly. Of these two, the inner planet was thought to be consistent with a rocky super-Earth, whereas the outer planet's low density implies it may have a substantial proportion of water ice<ref name="Jontof-Hutter" /> or a significant gas envelope, resembling a miniaturized gas giant (a gas dwarf).<ref name = "Kipping"/> The striking differences between these two planets have been hypothesized to be due to photoevaporation.<ref name = "Kipping"/> However, more recent observations as of 2022 have found similarly low densities for both planets c and d, suggesting that they are likely to be ocean worlds.<ref name="Piaulet2022"/><ref name="NASA2022"/> The mass of candidate Kepler-138e would be intermediate of Mars and Venus. While a radius could not be estimated for planet e, it is likely smaller than c and d and larger than b, which is consistent with an Earth-like composition.<ref name="Piaulet2022"/>
The three inner planets are too close to their star to be considered within the habitable zone,<ref name = "Briefing"/> while the likely planet Kepler-138e orbits near the inner edge of the habitable zone.<ref name="Piaulet2022"/>
{{OrbitboxPlanet begin | name = Kepler-138 | table_ref = <ref name="Piaulet2022"/> }} {{OrbitboxPlanet | exoplanet = b | mass_earth = {{val|0.07|0.02}} | radius_earth = {{val|0.64|0.02}} | period = {{val|10.3134|0.0003}} | semimajor = {{val|0.0753|0.0006}} | eccentricity = {{val|0.020|0.009}} | inclination = {{val|88.67|0.08}} }} {{OrbitboxPlanet | exoplanet = c | mass_earth = {{val|2.3|0.6|0.5}} | radius_earth = {{val|1.51|0.04}} | period = {{val|13.78150|0.00007|0.00009}} | semimajor = {{val|0.0913|0.0007}} | eccentricity = {{val|0.017|0.008|0.007}} | inclination = {{val|89.02|0.07}} }} {{OrbitboxPlanet | exoplanet = d | mass_earth = {{val|2.1|0.6|0.7}} | radius_earth = {{val|1.51|0.04}} | period = {{val|23.0923|0.0006}} | semimajor = {{val|0.1288|0.0010}} | eccentricity = {{val|0.010|0.005}} | inclination = {{val|89.04|0.04}} }} {{OrbitboxPlanet hypothetical | exoplanet = e | mass_earth = {{val|0.43|0.21|0.10}} | radius_earth = | period = {{val|38.230|0.006}} | semimajor = {{val|0.1803|0.0014}} | eccentricity = {{val|0.112|0.018|0.024}} | inclination = {{val|88.53|1.0}} }} {{Orbitbox end}}
==See also== * Hunt for Exomoons with Kepler * List of multiplanetary systems
==References== <references> <ref name="SIMBAD">{{cite simbad |title=Kepler-138 |access-date=17 December 2022}}</ref>
<ref name="GaiaDR3">{{Cite Gaia DR3|2102053446751282944}}</ref>
<ref name="kepler-catalogue">{{cite web|title=Kepler Input Catalog search result|publisher=Space Telescope Science Institute|url=http://archive.stsci.edu/kepler/kic10/search.php?kic_kepler_id=7603200&action=Search |access-date=4 March 2014}}</ref>
<ref name="NASAExoplanetArchive">{{cite web |url=https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-138 |title=Kepler-138 |website=NASA Exoplanet Archive |access-date=17 December 2022}}</ref>
<ref name="McQuillan2013">{{cite journal | title=Stellar Rotation Periods of The Kepler objects of Interest: A Dearth of Close-In Planets Around Fast Rotators | last1=McQuillan | first1=A. | last2=Mazeh | first2=T. | last3=Aigrain | first3=S. | journal=The Astrophysical Journal Letters | volume=775 | issue=1 | pages=L11 | year=2013 | arxiv=1308.1845 | bibcode=2013ApJ...775L..11M | doi=10.1088/2041-8205/775/1/L11 | s2cid=118557681 }}</ref>
<ref name = "Pineda">{{Cite journal | last1=Pineda | first1=Sebastian | title=Using High-resolution Optical Spectra to Measure Intrinsic Properties of Low-mass Stars: New Properties for KOI-314 and GJ 3470 | journal=The Astrophysical Journal| volume=767 | issue=1 | pages=28 | date=21 March 2013 | doi=10.1088/0004-637X/767/1/28 |arxiv = 1302.6231 |bibcode = 2013ApJ...767...28P | display-authors=etal | last2=Bottom | first2=Michael | s2cid=12541740 }}</ref>
<ref name = "Mann">{{Cite journal | last1=Mann | first1=Andrew | title=Spectro-thermometry of M Dwarfs and Their Candidate Planets: Too Hot, Too Cool, or Just Right? | journal=The Astrophysical Journal| volume=779 | issue=2 | pages=188 | date=4 December 2013 | doi=10.1088/0004-637X/779/2/188 |arxiv = 1311.0003 |bibcode = 2013ApJ...779..188M | display-authors=etal | last2=Gaidos | first2=Eric | hdl=2152/34640 | s2cid=119183731 }}</ref>
<ref name = "Kipping">{{Cite journal |last1=Kipping |first1=David |author-link=David Kipping |title=The Hunt for Exomoons with Kepler (HEK): IV. A Search for Moons around Eight M-Dwarfs |journal=The Astrophysical Journal |volume=784 |issue=1 |pages=28–41 |date=1 March 2014 |doi=10.1088/0004-637X/784/1/28 |arxiv=1401.1210 |bibcode=2014ApJ...784...28K |display-authors=2 |last2=Nesvorný |first2=D. |last3=Hartman |first3=J. |last4=Bakos |first4=G. Á. |last5=Schmitt |first5=A. R.|s2cid=119305398 }}</ref>
<ref name="Rowe">{{cite journal |title=Validation of Kepler's Multiple Planet Candidates. III: Light Curve Analysis & Announcement of Hundreds of New Multi-planet Systems |date=2014 |last1= Rowe |first1=Jason F. | arxiv=1402.6534 |display-authors=etal |doi=10.1088/0004-637X/784/1/45 |volume=784 |issue=1 |journal=The Astrophysical Journal |page=45 |bibcode=2014ApJ...784...45R|s2cid=119118620 }}</ref>
<ref name = "Jontof-Hutter">{{cite journal |last1=Jontof-Hutter |first1=D |title=Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing |journal=Nature |volume=522 |issue=7556 |pages=321–323 |date=18 June 2015 |doi=10.1038/nature14494 |arxiv=1506.07067 |bibcode=2015Natur.522..321J |display-authors=etal |last2=Rowe |first2=J |pmid=26085271|s2cid=205243944 }}</ref>
<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>
<!--<ref name="Almenara2018">{{Cite journal|arxiv=1805.12520 |last1=Almenara |first1=J. M |display-authors=1 |title=Absolute densities in exoplanetary systems: Photodynamical modelling of Kepler-138 |journal=Monthly Notices of the Royal Astronomical Society |volume=478 |issue=1 |pages=460–486 |last2=Díaz |first2=R. F |last3=Dorn |first3=C |last4=Bonfils |first4=X |last5=Udry |first5=S |bibcode=2018MNRAS.478..460A |doi=10.1093/mnras/sty1050 |year=2018 |s2cid=54614204 }}</ref>-->
<ref name="Piaulet2022">{{cite journal |last1=Piaulet |first1=Caroline |last2=Benneke |first2=Björn |display-authors=etal |date=15 December 2022 |title=Evidence for the volatile-rich composition of a 1.5-Earth-radius planet |journal=Nature Astronomy |volume= 7|issue= |page=206 |doi=10.1038/s41550-022-01835-4 |arxiv=2212.08477|bibcode=2023NatAs...7..206P |s2cid=254764810 }}</ref>
<ref name="NASA2022">{{cite web |url=https://exoplanets.nasa.gov/news/1717/two-super-earths-may-be-mostly-water/ |title=Two Super-Earths May Be Mostly Water |date=15 December 2022 |publisher=NASA |access-date=17 December 2022}}</ref>
<ref name="Roman1987">{{Cite constellation|Kepler-138}}</ref> </references>
==External links == *[https://web.archive.org/web/20140213051553/http://www.cfa.harvard.edu/HEK/ The Hunt for Exomoons with Kepler (HEK) project] - HEK. *[https://web.archive.org/web/20131105082102/http://www.nasa.gov/mission_pages/kepler/main/index.html Kepler Mission] – NASA. *[https://web.archive.org/web/20100527104316/http://kepler.nasa.gov/Mission/discoveries/ Kepler – Discoveries – Summary Table] – NASA. *[https://www.nytimes.com/interactive/science/space/keplers-tally-of-planets.html Kepler – Tally of Planets/interactive (2013)] – NYT. *[https://www.youtube.com/watch?v=9jNcfRsUK8o Video (10:47) - David Kipping - Press Briefing on First Earth-Mass Transiting Planet, KOI-314c (01/06/2014).]
{{Stars of Lyra}} {{Portal bar|Astronomy|Biology}} {{Sky|19|21|31.563|+|43|17|34.76}}
Category:Planetary systems with three confirmed planets Category:M-type main-sequence stars Category:Planetary transit variables Category:Lyra 0314