Kepler-138

Kepler-138, also known as KOI-314, is a red dwarf[3][7] located in the constellation Lyra, 218 light years from Earth.[1] 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.

KOI-314

Observation data
Epoch J2000      Equinox J2000
Constellation Lyra
Right ascension 19h 21m 31.5682s[1]
Declination +43° 17′ 34.6804″[1]
Apparent magnitude (V) 12.925 (Kepler band)[2]
Characteristics
Spectral type M0V[3]
Astrometry
Proper motion (μ) RA: −20.599±0.045[1] mas/yr
Dec.: 22.698±0.045[1] mas/yr
Parallax (π)14.9272 ± 0.0238[1] mas
Distance218.5 ± 0.3 ly
(67.0 ± 0.1 pc)
Absolute magnitude (MV)8.81 ± 0.28
Details
Mass0.468+0.030
−0.028[4] M☉
Radius0.54 ± 0.05[3] R☉
Luminosity (bolometric)0.0354[4] L☉
Luminosity (visual, LV)0.026 ± 0.006 L☉
Surface gravity (log g)4.74 ± 0.09[3] cgs
Temperature3726+44
−40[4] K
Metallicity [Fe/H]-0.28 ± 0.10[3] dex
Rotation19.394±0.013 days[5]
Rotational velocity (v sin i)~3[6] km/s
Age>1[6] Gyr
Other designations
Kepler-138, KIC 7603200, 2MASS J19213157+4317347
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data
KICdata

The star hosts three confirmed planets, including the lowest-mass planet with a measured mass and size discovered to date, Kepler-138b,[8] with a mass comparable to that of Mars. Kepler-138d is remarkable for being a gas dwarf[6] despite being lighter than Earth.

Nomenclature and history
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 phenomenon can also be responsible which is why the term planetary candidate is used.[9] By timing these dips, gravitational interactions were detected between two of the candidates,[6] allowing for a measurement of their masses and confirmation as real planets given that the masses were significantly below the deuterium burning limits.[10]

Following the acceptance of the discovery paper, the Kepler team provided an additional moniker for the system of "Kepler-138".[11] However, the planets were discovered by scientists outside of the Kepler team[6] and naming precedent belongs to the original discoverers (who refer to the star as KOI-314[6][12]), unless the discoverers provide formal agreement for a change of name.[13]

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.[2] If planets candidates are detected simultaneously, then the ordering follows the order of orbital periods from shortest to longest.[2] Following these rules, the first two candidate planets were detected simultaneously[14] and assigned the names KOI-314.01 and KOI-314.02, with respective orbital periods of 13.8 and 23.1 days.[14] Over a year later, a much smaller planet candidate was detected and assigned the name KOI-314.03,[15] despite being the shortest orbital period planet (period of 10.3 days) found to date.

Confirmed planets are conventionally assigned the designations b, c, d, etc. after the star's name.[16] The labels are assigned alphabetically in the order of discovery starting from b.[16] 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,[6] 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.[6]

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%.[17] 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.[17] Since KOI-314b and KOI-314c had been discovered in an earlier published article,[6] naming precedent belongs to the original paper and thus KOI-314b and c cannot be renamed without formal agreement by the discoverers.[13] However, KOI-314.03 was confirmed only in the later paper meaning this planet's name is given by the discoverers in the second paper, who use Kepler-138b. This has led to the unusual situation of having one star with two b planets in orbit and two different base-names for the orbiting planets.

Stellar characteristics

Kepler-138 is a red dwarf with approximately 57% the mass of and 54% the radius of the Sun.[3] It has a surface temperature of 3871 ± 58 K.[7] In comparison, the Sun has a surface temperature of 5778 K.[18] Kepler-138's apparent magnitude (bright it appears from Earth's perspective) is 12.925,[2] too dim to be seen with the naked eye.

Planetary system

All known planets 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' periods and relative diameters (compared to the host star) by monitoring each planet's transit of the star.[6][17]

Although the inner most planet has a size similar to Mars, Kepler-138c and d both have a radius of 1.2 Earth radii.[6][17] All of the planets are too close to their star to be considered within the habitable zone.[12] Although Kepler-138c and d have the same size, their masses and densities vary greatly. Of these two, the inner planet is consistent with a rocky super-Earth, whereas the outer planet's low density implies it may have a substantial proportion of water ice[8] or a significant gas envelope, resembling a miniaturized gas giant (a Gas dwarf).[6] The striking differences between these two planets have been hypothesized to be due photoevaporation.[6]

The Kepler-138 planetary system[4][6][8][17][19]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b 0.187 M⊕ 0.0760 ± 0.0033 10.3132068+0.00003440
−0.0000451		 0.0060 89.5 ± 0.5° 0.46+0.03
−0.03 R⊕
c 5.2 M⊕ 0.0922 ± 0.0040 13.78110882+0.00000750
−0.00000745		 0.0110 88.5 ± 0.2° 1.17+0.07
−0.07 R⊕
d 1.17 M⊕ 0.1301 ± 0.0056 23.0889203+0.0000232
−0.0000247		 0.0270 ± 0.0050 88.8 ± 0.1° 1.20+0.11
−0.10 R⊕

The KOI and Kepler letters for these planets don't match. KOI-314.01 and KOI-314.02 were confirmed by Kipping et al.,[6] but the Kepler team assigned the letters to Kepler-138c and Kepler-138d in their validation paper.[17]

See also

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. "Kepler Input Catalog search result". Space Telescope Science Institute. Retrieved 4 March 2014.
  3. Pineda, Sebastian; Bottom, Michael; et al. (21 March 2013). "Using High-resolution Optical Spectra to Measure Intrinsic Properties of Low-mass Stars: New Properties for KOI-314 and GJ 3470". The Astrophysical Journal. 767 (1): 28. arXiv:1302.6231. Bibcode:2013ApJ...767...28P. doi:10.1088/0004-637X/767/1/28. S2CID 12541740.
  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. McQuillan, A.; Mazeh, T.; Aigrain, S. (2013). "Stellar Rotation Periods of The Kepler objects of Interest: A Dearth of Close-In Planets Around Fast Rotators". The Astrophysical Journal Letters. 775 (1): L11. arXiv:1308.1845. Bibcode:2013ApJ...775L..11M. doi:10.1088/2041-8205/775/1/L11. S2CID 118557681.
  6. Kipping, David; Nesvorný, D.; et al. (1 March 2014). "The Hunt for Exomoons with Kepler (HEK): IV. A Search for Moons around Eight M-Dwarfs". The Astrophysical Journal. 784 (1): 28–41. arXiv:1401.1210. Bibcode:2014ApJ...784...28K. doi:10.1088/0004-637X/784/1/28. S2CID 119305398.
  7. Mann, Andrew; Gaidos, Eric; et al. (4 December 2013). "Spectro-thermometry of M Dwarfs and Their Candidate Planets: Too Hot, Too Cool, or Just Right?". The Astrophysical Journal. 779 (2): 188. arXiv:1311.0003. Bibcode:2013ApJ...779..188M. doi:10.1088/0004-637X/779/2/188. hdl:2152/34640. S2CID 119183731.
  8. Jontof-Hutter, D; Rowe, J; et al. (18 June 2015). "Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing". Nature. 522 (7556): 321–323. arXiv:1506.07067. Bibcode:2015Natur.522..321J. doi:10.1038/nature14494. PMID 26085271. S2CID 205243944.
  9. 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.
  10. "Working Group on Extrasolar Planets: Definition of a "Planet"". IAU position statement. 28 February 2003. Archived from the original on 16 September 2006. Retrieved 9 September 2006.
  11. NASA (27 January 2014). "Kepler – Discoveries – Summary Table". NASA. Retrieved 1 March 2014.
  12. David Kipping (26 January 2014). "David Kipping - Press Briefing on First Earth-Mass Transiting Planet, KOI-314c". YouTube. Retrieved 1 March 2014.
  13. "Public Naming of Planets and Planetary Satellites" (PDF). International Astronomical Union. Retrieved 19 August 2013.
  14. Borucki, William J.; Koch, David G.; Batalha, Natalie; Brown, Timothy M.; Bryson, Stephen T.; Caldwell, Douglas; Christensen-Dalsgaard, Jørgen; Cochran, William D.; Devore, Edna; Dunham, Edward W.; Gautier, Thomas N.; Geary, John C.; Gilliland, Ronald; Gould, Alan; Howell, Steve B.; Jenkins, Jon M.; Latham, David W.; Lissauer, Jack J.; Marcy, Geoffrey W.; Rowe, Jason; Sasselov, Dimitar; Boss, Alan; Charbonneau, David; Ciardi, David; Doyle, Laurance; Dupree, Andrea K.; Ford, Eric B.; Fortney, Jonathan; Holman, Matthew J.; et al. (29 June 2011). "Characteristics of Planetary Candidates Observed by Kepler. II. Analysis of the First Four Months of Data". The Astrophysical Journal. 736 (1): 19. arXiv:1102.0541. Bibcode:2011ApJ...736...19B. doi:10.1088/0004-637X/736/1/19. S2CID 15233153.
  15. Batalha, Natalie; Rowe, Jason F.; Barclay, Thomas; Burke, Christopher J.; Caldwell, Douglas A.; Christiansen, Jessie L.; Mullally, Fergal; Thompson, Susan E.; Brown, Timothy M.; Dupree, Andrea K.; Fabrycky, Daniel C.; Ford, Eric B.; Fortney, Jonathan J.; Gilliland, Ronald L.; Isaacson, Howard; Latham, David W.; Marcy, Geoffrey W.; Quinn, Samuel N.; Ragozzine, Darin; Shporer, Avi; Borucki, William J.; Ciardi, David R.; Gautier, Thomas N.; Haas, Michael R.; Jenkins, Jon M.; Koch, David G.; Lissauer, Jack J.; Rapin, William; Basri, Gibor S.; et al. (5 February 2013). "Planetary Candidates Observed by Kepler. III. Analysis of the First 16 Months of Data". The Astrophysical Journal Supplement. 204 (2): 24. arXiv:1202.5852. Bibcode:2013ApJS..204...24B. doi:10.1088/0067-0049/204/2/24. S2CID 19023502.
  16. 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].
  17. Rowe, Jason F.; et al. (2014). "Validation of Kepler's Multiple Planet Candidates. III: Light Curve Analysis & Announcement of Hundreds of New Multi-planet Systems". The Astrophysical Journal. 784 (1): 45. arXiv:1402.6534. Bibcode:2014ApJ...784...45R. doi:10.1088/0004-637X/784/1/45. S2CID 119118620.
  18. Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
  19. Almenara, J. M; et al. (2018). "Absolute densities in exoplanetary systems: Photodynamical modelling of Kepler-138". Monthly Notices of the Royal Astronomical Society. 478 (1): 460–486. arXiv:1805.12520. Bibcode:2018MNRAS.478..460A. doi:10.1093/mnras/sty1050. S2CID 54614204.

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