K2-18

Artist's impression of the K2-18 system, with K2-18 on left, K2-18b on right, and K2-18c between.
Credit: ESA/Hubble
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
Epoch J2000.0      Equinox J2000.0
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 mas/yr[1]
Dec.: −133.007 mas/yr[1]
Parallax (π)26.2469 ± 0.0266 mas[1]
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 2 3 4 5 6 Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. 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. 1 2 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: 25. arXiv:1503.07866. Bibcode:2015ApJ...809...25M. doi:10.1088/0004-637X/809/1/25. S2CID 33348734.
  4. 1 2 3 4 5 Cloutier, Ryan; et al. (7 January 2019). "Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES". Astronomy & Astrophysics. 621 A49: A49. arXiv:1810.04731. Bibcode:2019A&A...621A..49C. doi:10.1051/0004-6361/201833995. S2CID 118828975.
  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: 72. arXiv:1701.00588. Bibcode:2017ApJ...837...72M. doi:10.3847/1538-4357/aa56c7. S2CID 25502982.
  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: 187. arXiv:1610.07249. Bibcode:2017ApJ...834..187B. doi:10.3847/1538-4357/834/2/187. S2CID 12988198.
  7. 1 2 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. Bibcode:2019RNAAS...3..189G. doi:10.3847/2515-5172/ab6086. ISSN 2515-5172.
  8. ↑ "HABITABLE EXOPLANETS CATALOG". UPR. 7 March 2016. Retrieved 12 September 2019.
  9. ↑ "EPIC 201912552 b reality check drewexmachina 11-22-2015". Drew ExMachina. 7 March 2016. Retrieved 12 September 2019.
  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; Greene, Thomas P.; Dressing, Courtney; Dragomir, Diana; Howard, Andrew W.; McCullough, Peter R.; Kempton, Eliza M.-R.; Fortney, Jonathan J.; Fraine, Jonathan (December 2019). "Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b". The Astrophysical Journal Letters. 887 (1): L14. arXiv:1909.04642. Bibcode:2019ApJ...887L..14B. doi:10.3847/2041-8213/ab59dc. ISSN 2041-8205. S2CID 209324670.
  12. 1 2 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. arXiv:2309.05566. Bibcode:2023ApJ...956L..13M. doi:10.3847/2041-8213/acf577.
  13. ↑ Cloutier, R.; Astudillo-Defru, N.; 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. arXiv:1707.04292. Bibcode:2017A&A...608A..35C. doi:10.1051/0004-6361/201731558.
  14. ↑ Sarkis, Paula; Henning, Thomas; 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. arXiv:1805.00830. Bibcode:2018AJ....155..257S. doi:10.3847/1538-3881/aac108.
  15. 1 2 Radica, Michael; Artigau, Étienne; 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. arXiv:2210.08078. Bibcode:2022MNRAS.517.5050R. doi:10.1093/mnras/stac3024.
  16. 1 2 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. arXiv:2005.10318. Bibcode:2020MNRAS.494.5082G. doi:10.1093/mnras/staa1110. S2CID 218763252.
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