K2-18b

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K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 38 parsecs away from Earth.[1][2] The planet, initially discovered with the Kepler space telescope, is about eight times the mass of Earth, and is thus classified as a super Earth or a Mini-Neptune, and, as well, may be considered a hycean planet.[3] It has a 33-day orbit within the star's habitable zone.

In September 2019, two independent research studies, combining data from the Kepler space telescope, the Spitzer Space Telescope, and the Hubble Space Telescope, concluded that there are significant amounts of water vapor in its atmosphere, a first for an exoplanet in the habitable zone.[4][5][6]

Discovery

K2-18b was identified as part of the Kepler space telescope program, one of over 1,200 exoplanets discovered during the "Second Light" K2 mission.[7] The discovery of K2-18b was made in 2015, orbiting a red dwarf star (now known as K2-18) with a stellar spectral type of M2.8 about 38|parsec|light-year|sigfig=3|order=flip}} from Earth. The planet was detected through variations in the star's light curve caused by the transit of the planet in front of the star as seen from Earth.[8][9] The planet was designated "K2-18b" as it was the eighteenth planet discovered during the K2 mission. The predicted relatively low contrast between the planet and its host star would make it easier to observe K2-18b's atmosphere in the future.[8]

In 2017, data from the Spitzer Space Telescope confirmed that K2-18b orbits in the habitable zone around K2-18 with a 33-day period, short enough to allow for observations of multiple K2-18b orbital cycles and improving the statistical significance of the signal. This led to widespread interest in continued observations of K2-18b.[10]

Later studies on K2-18b using the High Accuracy Radial Velocity Planet Searcher (HARPS) and the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs (CARMENES) instruments also identified a likely second exoplanet, K2-18c, with an estimated mass of 5.62 +/- 0.84 Earth mass in a tighter, 9-day orbit,[1] but this additional planet has not yet been confirmed, and may instead be due to stellar activity.[11]

Location

File:StarK2-18 Location StellariumScreenshot.png
The location of the star K2-18 in the sky, marked by a white circle.

K2-18 is in the constellation of Leo, but outside its lion asterism.[12] When first discovered, K2-18's distance from Earth was estimated to be 34 parsecs.[8] However, more precise data from the Gaia star mapping project has shown K2-18 to be at a distance of 38.025 +/- 0.079 parsecs. This improved distance measurement helped to refine the properties of the exoplanetary system.[1]

Physical characteristics

K2-18b orbits K2-18 at about 0.1429 astronomical units, which lies within the calculated habitable zone for the red dwarf, 0.12 - 0.25 astronomical units.[6] The exoplanet has an orbital period of about 33 days,[10] which suggests it is tidally locked, with the same face to its host star.[13] The planet's equilibrium temperature is estimated to be around 265 degrees Kelvin,[1] due to its stellar irradiance of approximately 94% of Earth's.[10] K2-18b is estimated to have a radius of 2.279 +/- 0.025 Earth radius and a mass of 8.63 +/- 1.35 Earth mass, based on analysis using HARPS and CARMENES instruments as well as followup observations from Spitzer.[1][10] It was initially considered a mini-Neptune on its 2015 discovery,[8] but improved data on K2-18b has classified it as a super-Earth.[10] A later study from 2019 classified the planet as a sub-Neptune.[14][15]

  • K2-18 Star System Video.webm

    Artist's impression of the K2-18 star system

  • K2-18 system.jpg

    Diagram of the K2-18 planetary system, showing the orbits of K2-18b and the unconfirmed candidate K2-18c, and the star's habitable zone

Planetary atmosphere

A comparison of K2-18b's size, orbit, and other features to other detected exoplanets suggests that the planet could support an atmosphere that contains additional gases besides hydrogen and helium.[16]

Further studies using the Hubble Space Telescope were performed, corroborating the results of the Kepler and Spitzer observations and allowing additional measurements of the planet's atmosphere. Two separate analyses by researchers at Université de Montréal and University College London (UCL) of the Hubble data were published in 2019. Both examined spectra of starlight passing through the planet's atmosphere during transits, finding that K2-18b has a hydrogen–helium atmosphere with a high concentration of water vapor, which could range from between 0.01% to 12.5%, up to between 20% and 50%, depending on what other gaseous species are present in the atmosphere. At the upper concentration levels, the water vapor would be sufficiently high to form clouds.[5][6][17] The UCL-led study was published on 11 September 2019 in the journal Nature Astronomy; the study led from the Université de Montréal was posted one day earlier on the preprint server arXiv.org and later published in The Astrophysical Journal Letters.[13] The UCL-led analysis detected water with a statistical significance of 3.6 standard deviations, equivalent to a confidence level of 99.97%.[6] This was the first super-Earth exoplanet within a star's habitable zone whose atmosphere was detected,[6] and the first discovery of water in a habitable-zone exoplanet.[4][5] Water had previously been detected in the atmospheres of non-habitable-zone exoplanets such as HD 209458 b, XO-1b, WASP-12b, WASP-17b, and WASP-19b.[18][19][20]

Astronomers emphasised that the discovery of water in the atmosphere of K2-18b does not mean the planet can support life or is even habitable, as it probably lacks any solid surface or an atmosphere that can support life.[4] Nevertheless, finding water in a habitable zone exoplanet helps understand how planets are formed.[4] A study led by astronomers from the University of Cambridge considered the interior structure of the planet and found a range of possible solutions, from a rocky core with a thick hydrogen envelope to a planet primarily made up of water with a thinner atmosphere. A subset of these solutions could allow for liquid water on the surface of the planet, albeit at temperatures and pressures higher than STP.[21] K2-18b is now expected to be observed with the James Webb Space Telescope, launched in 2021, and the ARIEL space telescope, due to launch in 2029. Both will carry instruments designed to determine the composition of exoplanet atmospheres.[5]

The detailed simulation of planetary spectrum in 2020 has indicated the 1.4 µm absorption band attributed previously to water may actually be due to methane. The water vapor spectral signatures would not be dominant for cool (below 600 K) planets.[22][23] In 2021, it was further deemed the alleged water absorption spectral feature may be coming from time-variable star spots of the parent star, not from the planetary atmosphere.[24]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Cloutier, Ryan et al. (7 January 2019). "Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES". Astronomy & Astrophysics 621: A49. arXiv:1810.04731. Bibcode 2019A&A...621A..49C. doi:10.1051/0004-6361/201833995. 
  2. ↑ University of Cambridge (26 February 2020). "Large exoplanet could have the right conditions for life". Phys.org. https://phys.org/news/2020-02-large-exoplanet-conditions-life.html. 
  3. ↑ Cohen, Liz (27 August 2021). "Scientists may find life on Earth-like planets covered in oceans within the next few years". CBS News. https://www.cbsnews.com/news/hycean-exoplanets-extraterrestrial-life-earth-like-planets-oceans/. Retrieved 28 August 2021. 
  4. ↑ 4.0 4.1 4.2 4.3 Ghosh, Pallab (12 September 2019). "Water found for first time on 'potentially habitable' planet". BBC News. https://www.bbc.com/news/science-environment-49648746. 
  5. ↑ 5.0 5.1 5.2 5.3 Greshko, Michael (11 September 2019). "Water found on a potentially life-friendly alien planet". National Geographic. https://www.nationalgeographic.com/science/2019/09/first-water-found-in-habitable-exoplanets-atmosphere-hubble-kepler-k2-18b/. 
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Tsiaras, Angelos et al. (11 September 2019). "Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b". Nature Astronomy 3 (12): 1086–1091. arXiv:1909.05218. Bibcode 2019NatAs.tmp..451T. doi:10.1038/s41550-019-0878-9. 
  7. ↑ "NASA's Kepler Mission Announces Largest Collection of Planets Ever Discovered" (Press release). NASA. 10 May 2016. https://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered/. 
  8. ↑ 8.0 8.1 8.2 8.3 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. arXiv:1503.07866. Bibcode 2015ApJ...809...25M. doi:10.1088/0004-637X/809/1/25. 
  9. ↑ Foreman-Mackey, Daniel et al. (18 June 2015). "A systematic search for transiting planets in the K2 data". The Astrophysical Journal 806 (2): 215. arXiv:1502.04715. Bibcode 2015ApJ...806..215F. doi:10.1088/0004-637x/806/2/215. 
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Benneke, Björn et al. (12 January 2017). "Spitzer Observations Confirm and Rescue the Habitable-zone Super-earth K2-18b for Future Characterization". The Astrophysical Journal 834 (2): 187. arXiv:1610.07249. Bibcode 2017ApJ...834..187B. doi:10.3847/1538-4357/834/2/187. 
  11. ↑ Sarkis, Paula et al. (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. 
  12. ↑ "K2-18 -- High proper-motion Star". SIMBAD. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=k2-18. 
  13. ↑ 13.0 13.1 Wall, Mike (2019-09-11). "The Water Vapor Find on 'Habitable' Exoplanet K2-18 b Is Exciting — But It's No Earth Twin.". Space.com. https://www.space.com/alien-planet-k2-18b-water-vapor-not-earth-twin.html. "Tsiaras and his colleagues published their results today (Sept. 11) in the journal Nature Astronomy. The other research team, led by Björn Benneke of the Université de Montréal, posted its paper on the online preprint site arXiv.org Tuesday. The study by Benneke et al. has not yet been peer-reviewed." 
  14. ↑ "Today's Forecast for K2-18b: Cloudy with a Chance of Rain?" (in en-US). https://aasnova.org/2020/01/31/todays-forecast-for-k2-18b-cloudy-with-a-chance-of-rain/. 
  15. ↑ 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 887 (1): L14. arXiv:1909.04642. Bibcode 2019ApJ...887L..14B. doi:10.3847/2041-8213/ab59dc. ISSN 2041-8205. 
  16. ↑ Cloutier, R. et al. (5 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 (35): A35. arXiv:1707.04292. Bibcode 2017A&A...608A..35C. doi:10.1051/0004-6361/201731558. https://www.aanda.org/articles/aa/full_html/2017/12/aa31558-17/aa31558-17.html. Retrieved 11 September 2019. 
  17. ↑ Grossman, Lisa (2019-09-11). "This may be the first known exoplanet with rain and clouds of water droplets". ScienceNews. https://www.sciencenews.org/article/first-known-exoplanet-rain-clouds-water-droplets. 
  18. ↑ "Hubble Traces Subtle Signals of Water on Hazy Worlds". NASA. 3 December 2013. http://www.nasa.gov/content/goddard/hubble-traces-subtle-signals-of-water-on-hazy-worlds/. 
  19. ↑ Deming, D. et al. (2013). "Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope". The Astrophysical Journal 774 (2): 95. arXiv:1302.1141. Bibcode 2013ApJ...774...95D. doi:10.1088/0004-637X/774/2/95. 
  20. ↑ Mandell, A. M. et al. (2013). "Exoplanet Transit Spectroscopy Using WFC3: WASP-12 b, WASP-17 b, and WASP-19 b". The Astrophysical Journal 779 (2): 128. arXiv:1310.2949. Bibcode 2013ApJ...779..128M. doi:10.1088/0004-637X/779/2/128. 
  21. ↑ Madhusudhan, Nikku et al. (March 2020). "The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b". The Astrophysical Journal Letters 891 (1): L7. arXiv:2002.11115. Bibcode 2020ApJ...891L...7M. doi:10.3847/2041-8213/ab7229. 
  22. ↑ Bézard, Bruno; Charnay, Benjamin; Blain, Doriann (2020), Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b, arXiv:2011.10424 
  23. ↑ Blain, D.; Charnay, B.; Bézard, B. (2020), "1D atmospheric study of the temperate sub-Neptune K2-18b", Astronomy & Astrophysics, arXiv:2011.10459, doi:10.1051/0004-6361/202039072 
  24. ↑ Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum, 2021, arXiv:2109.14608 

External links

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