Astronomy:K2-18

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Short description: Red dwarf star in the constellation Leo
K2-18
Exoplanet K2-18 b (illustration) (weic2321a).tiff
Artist's impression of the K2-18 system, with K2-18 on left, K2-18b on right, and K2-18c between.
Credit: ESA/Hubble
Leo constellation map.svg
Red circle.svg
Approximate two-dimensional location of the star (in red circle); Sigma Leonis is the nearest bright star, which is in a southerly direction, and the boundary of Virgo is similarly far.
Observation data
Equinox J2000.0]] (ICRS)
Constellation Leo
Right ascension  11h 30m 14.51774s[1]
Declination +07° 35′ 18.2553″[1]
Apparent magnitude (V) 13.50[2]
Characteristics
Evolutionary stage Red dwarf
Spectral type M2.8[3]
Astrometry
Radial velocity (Rv)0.02±0.52[1] km/s
Proper motion (μ) RA: −80.479[1] mas/yr
Dec.: −133.007[1] mas/yr
Parallax (π)26.2469 ± 0.0266[1] mas
Distance124.3 ± 0.1 ly
(38.10 ± 0.04 pc)
Details
Mass0.495±0.004[4] M☉
Radius0.469±0.010[4] R☉
Luminosity0.0234[5] L☉
Temperature3,503±60[4] K
Metallicity [Fe/H]0.123±0.157[6] dex
Rotation39.6±0.9 d[7]
Age2.4±0.6[7] Gyr
Other designations
EPIC 201912552, UCAC4 488-054338, 2MASS J11301450+0735180
Database references
SIMBADdata

K2-18, also known as EPIC 201912552, is a red dwarf star with two planetary companions located 124 light-years (38 parsecs)[4] from Earth, in the constellation of Leo.

Planetary system

The star has a transiting exoplanet, called K2-18b, a super-Earth located within the habitable zone of K2-18.[8][9] It was discovered in 2015 by the Kepler space telescope in its K2 mission.[3] It is the first exoplanet in the habitable zone, albeit a hydrogen-rich sub-Neptune,[10] to have its atmosphere characterized; initially thought to contain water vapor,[11] more recent observations have instead detected methane and carbon dioxide.[12] The presence of these molecules and non-detection of ammonia is consistent with predictions for a hycean planet.[12]

A second, non-transiting planet, K2-18c, was discovered in 2017 by radial velocity with HARPS.[13] This planet was challenged by another team with CARMENES data,[14] but its existence was reaffirmed by the discovery team based on both HARPS and CARMENES data.[4] This planet has also been confirmed by a later independent study.[15] System tidal simulation suggests that K2-18c is a gas-rich, Neptune-like planet, similar to K2-18b.[16]

The K2-18 planetary system[16]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
c ≥5.62±0.84 M⊕ 0.0670 ± 0.0002 9.2072±0.0065[15] <0.2 — —
b 8.63±1.35 M⊕ 0.1591±0.0004 32.94488±0.00281 — — 2.711±0.065 R⊕

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  2. ↑ Zacharias, N.; Finch, C. T.; Girard, T. M.; Henden, A.; Bartlett, J. L.; Monet, D. G.; Zacharias, M. I. (2012). "VizieR Online Data Catalog: UCAC4 Catalogue (Zacharias+, 2012)". VizieR On-line Data Catalog. Bibcode: 2012yCat.1322....0Z. 
  3. ↑ 3.0 3.1 Montet, Benjamin T. et al. (5 August 2015). "Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation". The Astrophysical Journal 809 (1): 25. doi:10.1088/0004-637X/809/1/25. Bibcode: 2015ApJ...809...25M. 
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Cloutier, Ryan et al. (7 January 2019). "Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES". Astronomy & Astrophysics 621: A49. doi:10.1051/0004-6361/201833995. Bibcode: 2019A&A...621A..49C. 
  5. ↑ Martinez, Arturo O. et al. (2017). "Stellar and Planetary Parameters for K2's Late-type Dwarf Systems from C1 to C5". The Astrophysical Journal 837 (1): 72. doi:10.3847/1538-4357/aa56c7. Bibcode: 2017ApJ...837...72M. 
  6. ↑ Benneke, Björn et al. (2017). "Spitzer Observations Confirm and Rescue the Habitable-zone Super-Earth K2-18b for Future Characterization". The Astrophysical Journal 834 (2): 187. doi:10.3847/1538-4357/834/2/187. Bibcode: 2017ApJ...834..187B. 
  7. ↑ 7.0 7.1 Guinan, Edward F.; Engle, Scott G. (2019-12-12). "The K2-18b Planetary System: Estimates of the Age and X-UV Irradiances of a Habitable Zone "Wet" Sub-Neptune Planet". Research Notes of the AAS 3 (12): 189. doi:10.3847/2515-5172/ab6086. ISSN 2515-5172. Bibcode: 2019RNAAS...3..189G. 
  8. ↑ "HABITABLE EXOPLANETS CATALOG". UPR. 7 March 2016. http://phl.upr.edu/projects/habitable-exoplanets-catalog. 
  9. ↑ "EPIC 201912552 b reality check drewexmachina 11-22-2015". Drew ExMachina. 7 March 2016. http://www.drewexmachina.com/2015/05/12/habitable-planet-reality-check-epic-201912552b/. 
  10. ↑ No, the Exoplanet K2-18b Is Not Habitable
  11. ↑ Benneke, Björn; Wong, Ian; Piaulet, Caroline; Knutson, Heather A.; Lothringer, Joshua; Morley, Caroline V.; Crossfield, Ian J. M.; Gao, Peter et al. (December 2019). "Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b" (in en). The Astrophysical Journal Letters 887 (1): L14. doi:10.3847/2041-8213/ab59dc. ISSN 2041-8205. Bibcode: 2019ApJ...887L..14B. 
  12. ↑ 12.0 12.1 Madhusudhan, Nikku; Sarkar, Subhajit; Constantinou, Savvas; Holmberg, Måns; Piette, Anjali A. A.; Moses, Julianne I. (1 October 2023). "Carbon-bearing Molecules in a Possible Hycean Atmosphere". The Astrophysical Journal Letters 956 (1): L13. doi:10.3847/2041-8213/acf577. Bibcode: 2023ApJ...956L..13M. 
  13. ↑ Cloutier, R. et al. (December 2017). "Characterization of the K2-18 multi-planetary system with HARPS. A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit". Astronomy & Astrophysics 608: A35. doi:10.1051/0004-6361/201731558. Bibcode: 2017A&A...608A..35C. 
  14. ↑ Sarkis, Paula et al. (June 2018). "The CARMENES Search for Exoplanets around M Dwarfs: A Low-mass Planet in the Temperate Zone of the Nearby K2-18". The Astronomical Journal 155 (6): 257. doi:10.3847/1538-3881/aac108. Bibcode: 2018AJ....155..257S. 
  15. ↑ 15.0 15.1 Radica, Michael et al. (December 2022). "Revisiting radial velocity measurements of the K2-18 system with the line-by-line framework". Monthly Notices of the Royal Astronomical Society 517 (4): 5050–5062. doi:10.1093/mnras/stac3024. Bibcode: 2022MNRAS.517.5050R. 
  16. ↑ 16.0 16.1 Ferraz-Mello, S.; Gomes, G. O. (2020). "Tidal evolution of exoplanetary systems hosting potentially habitable exoplanets. The cases of LHS-1140 b-c and K2-18 b-c". Monthly Notices of the Royal Astronomical Society 494 (4): 5082–5090. doi:10.1093/mnras/staa1110. Bibcode: 2020MNRAS.494.5082G. 

Coordinates: Sky map 11h 30m 14.518s, +07° 35′ 18.257″


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