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| {{Short description|Mini-Neptune orbiting the red dwarf K2-18}} | |||
| {{Use dmy dates|date=September 2019}} | |||
| {{Infobox planet | |||
| | extrasolarplanet = yes | |||
| | name = K2-18b | |||
| | image = File:Esa-hubble-k2-18a impression.jpg | |||
| | caption = Artist's impression of K2-18b (right) orbiting red dwarf K2-18 (left). The exoplanet ] is shown between them. | |||
| <!-- DISCOVERY --> | |||
| | discovery_ref = <ref name="discovery" /> | |||
| | discovery_site = ] | |||
| | discovered = 2015 | |||
| | discovery_method = ] | |||
| <!-- DESIGNATIONS --> | |||
| <!-- ORBITAL --> | |||
| | orbit_ref = <ref name=sarkis2018>{{cite journal |doi=10.3847/1538-3881/aac108 |title=The CARMENES Search for Exoplanets around M Dwarfs: A Low-mass Planet in the Temperate Zone of the Nearby K2-18 |journal=] |volume=155 |issue=6 |pages=257 |year=2018 |last1=Sarkis |first1=Paula |last2=Henning |first2=Thomas |last3=Kürster |first3=Martin |last4=Trifonov |first4=Trifon |last5=Zechmeister |first5=Mathias |last6=Tal-Or |first6=Lev |last7=Anglada-Escudé |first7=Guillem |last8=Hatzes |first8=Artie P. |last9=Lafarga |first9=Marina |last10=Dreizler |first10=Stefan |last11=Ribas |first11=Ignasi |last12=Caballero |first12=José A. |last13=Reiners |first13=Ansgar |last14=Mallonn |first14=Matthias |last15=Morales |first15=Juan C. |last16=Kaminski |first16=Adrian |last17=Aceituno |first17=Jesús |last18=Amado |first18=Pedro J. |last19=Béjar |first19=Victor J. S. |last20=Hagen |first20=Hans-Jürgen |last21=Jeffers |first21=Sandra |last22=Quirrenbach |first22=Andreas |last23=Launhardt |first23=Ralf |last24=Marvin |first24=Christopher |last25=Montes |first25=David |bibcode=2018AJ....155..257S |arxiv=1805.00830 |s2cid=73533100 | display-authors = 1 }}</ref> | |||
| | apsis = astron | |||
| | semimajor = {{line-height|2|{{val|0.1429|+0.0060|-0.0065|ul=au}}<br />{{convert|0.1429|AU|km|disp=out}}}} | |||
| | eccentricity = {{val|0.20|0.08}} | |||
| | period = {{val|32.939623|+0.000095|-0.000100|ul=d}} | |||
| | inclination = {{val|89.5785|+0.0079|-0.0088|u=deg}} | |||
| | arg_peri = {{val|-0.10|+0.81|-0.59|u=]}} ({{val|-5.73|+46.4|-33.8|u=])}} | |||
| | semi-amplitude = {{val|3.55|+0.57|-0.58|u=m/s}} | |||
| | star = ] | |||
| <!-- PHYS CHARS --> | |||
| | mean_radius = {{val|2.610|0.087|ul=R_Earth}}<ref name="benneke2019" /> | |||
| | mass = {{val|8.63|1.35|ul=M_Earth}}<ref name="Cloutier2019">{{cite journal |title=Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES |journal=] |volume=621 |at=A49 |date=7 January 2019 |doi=10.1051/0004-6361/201833995 |arxiv=1810.04731 |first1=Ryan |last1=Cloutier |first2=Nicola |last2=Astudillo-Defru |first3=René |last3=Doyon |first4=Xavier |last4=Bonfils |first5=José M. |last5=Almenara |first6=François |last6=Bouchy |first7=Xavier |last7=Delfosse |first8=Thierry |last8=Forveille |first9=Christophe |last9=Lovis |first10=Michel |last10=Mayor |first11=Kristen |last11=Menou |first12=Felipe |last12=Murgas |first13=Francesco |last13=Pepe |first14=Nuno C. |last14=Santos |first15=Stéphane |last15=Udry |first16=Anaël |last16=Wünsche |display-authors=1 |bibcode=2019A&A...621A..49C |s2cid=118828975 }}</ref> | |||
| | density = {{val|{{#expr: 8.63 * 5.9722e24 / (4/3 * pi * (2.610 * 6.3781e6)^3) / 1000 round 2 }}|u=g/cm3}} | |||
| | surface_grav = ≤{{val|{{#expr: 8.63 / 2.279^2 round 2 }}|u=]}} | |||
| | single_temperature = {{convert|265|+/-|5|K|C|0|lk=on}}<ref name="Cloutier2019" /> | |||
| <!-- ATMOSPHERE --> | |||
| <!-- NOTES --> | |||
| }} | |||
| '''K2-18b''', also known as '''EPIC 201912552 b''', is an ] orbiting the ] ], located {{Convert|38|pc|ly|sigfig=3|order=flip|lk=on}} away from Earth.<ref name="Cloutier2019" /><ref name="PHYS-20200226">{{cite news |author=University of Cambridge |title=Large exoplanet could have the right conditions for life |url=https://phys.org/news/2020-02-large-exoplanet-conditions-life.html |date=26 February 2020 |work=] |access-date=26 February 2020 |author-link=University of Cambridge }}</ref> The planet, initially discovered with the ], is about eight times the mass of Earth, and is thus classified as a ] or a ], and, as well, may be considered a ].<ref name="CBS-20210827">{{cite news |last=Cohen |first=Liz |title=Scientists may find life on Earth-like planets covered in oceans within the next few years |url=https://www.cbsnews.com/news/hycean-exoplanets-extraterrestrial-life-earth-like-planets-oceans/ |date=27 August 2021 |work=] |accessdate=28 August 2021 }}</ref> It has a 33-day orbit within the star's ]. | |||
| '''K12-8 b''' new article content ... | |||
| In September 2019, two independent research studies, combining data from the Kepler space telescope, the ], and the ], concluded that there are significant amounts of ] in its atmosphere, a first for an exoplanet in the habitable zone.<ref name="bbc">{{Cite web|last=Ghosh|first=Pallab|url=https://www.bbc.com/news/science-environment-49648746|title=Water found for first time on 'potentially habitable' planet|work=]|date=12 September 2019|access-date=13 September 2019|archive-url=https://web.archive.org/web/20190912215519/https://www.bbc.com/news/science-environment-49648746|archive-date=12 September 2019|url-status=live}}</ref><ref name="natgeo" /><ref name="tsiaras" /> | |||
| == |
== Star == | ||
| K2-18b was identified as part of the Kepler space telescope program, one of over 1,200 exoplanets discovered during the ].<ref>{{Cite press release | url = https://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered/ | title = NASA's Kepler Mission Announces Largest Collection of Planets Ever Discovered | date = 10 May 2016 | publisher = ] | access-date = 11 September 2019 | archive-url = https://web.archive.org/web/20190906045942/https://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered/ | archive-date = 6 September 2019 | url-status = live }}</ref> The discovery of K2-18b was made in 2015, orbiting a ] star (now known as K2-18) with a ] of M2.8 about {{Convert|38|parsec|light-year|sigfig=3|order=flip}} from Earth. The planet was detected through variations in the star's ] caused by the ] of the planet in front of the star as seen from Earth.<ref name="discovery">{{Cite journal | title = Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation | first1 = Benjamin T. | last1 = Montet | first2= Timothy D. |last2= Morton |first3= Daniel | last3= Foreman-Mackey | first4= John Asher | last4= Johnson | first5= David W. |last5= Hogg | first6= Brendan P. | last6 = Bowler | first7 = David W. | last7= Latham | first8= Allyson |last8= Bieryla | first9= Andrew W. |last9= Mann | journal = ] | volume = 809 | issue = 1 | pages = 25| date = 5 August 2015 | doi = 10.1088/0004-637X/809/1/25 | arxiv = 1503.07866 | display-authors=1 | bibcode = 2015ApJ...809...25M| s2cid = 33348734 }}</ref><ref name="Foreman">{{Cite journal | title = A systematic search for transiting planets in the K2 data | first1= Daniel | last1= Foreman-Mackey |first2 = Benjamin T. | last2 = Montet | first3= David W. |last3= Hogg | first4= Timothy D. |last4= Morton | first5= Dun | last5= Wang | first6= Bernhard | last6 = Schoelkopf | journal = ] | volume = 806 | issue = 2 | pages = 215| date = 18 June 2015 | doi = 10.1088/0004-637x/806/2/215 | arxiv = 1502.04715 | display-authors=1 | bibcode = 2015ApJ...806..215F| s2cid= 34456803 }}</ref> 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.<ref name="discovery" /> | |||
| A number of planets have been discovered around ] stars, including around ], ] and ] where they are located within the ] of the stars. The small mass, size and low temperatures of these stars make it easier to characterize these planets.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=1}} | |||
| In 2017, data from the Spitzer Space Telescope confirmed that K2-18b orbits in the ] 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.<ref name="spitzer">{{Cite journal | title = Spitzer Observations Confirm and Rescue the Habitable-zone Super-earth K2-18b for Future Characterization | journal = ] | volume = 834 | issue = 2 | pages = 187 | date = 12 January 2017 | doi = 10.3847/1538-4357/834/2/187 | first1 = Björn | last1 = Benneke | first2 = Michael | last2 = Werner | first3 =Erik | last3 = Petigura | first4 =Heather | last4= Knutson | first5 =Courtney | last5 = Dressing | first6 =Ian J. M. | last6 = Crossfield | first7 =Joshua E. | last7 = Schlieder | first8 =John | last8 = Livingston | first9 =Charles| last9 = Beichman | first10 =Jessie | last10 = Christiansen | first11 =Jessica | last11 = Krick | first12 =Varoujan| last12 = Gorjian | first13 =Andrew W. | last13 = Howard | first14 =Evan | last14 = Sinukoff | first15 =David R. | last15 = Ciardi6| first16 =Rachel L. | last16 = Akeson | display-authors=1 | arxiv = 1610.07249| bibcode = 2017ApJ...834..187B| s2cid = 12988198 }}</ref> | |||
| == Physical properties == | |||
| Later studies on K2-18b using the ] (HARPS) and the ] (CARMENES) instruments also identified a likely second exoplanet, K2-18c, with an estimated mass of {{Val|5.62|0.84|ul=M_Earth}} in a tighter, 9-day orbit,<ref name="Cloutier2019" /> but this additional planet has not yet been confirmed, and may instead be due to stellar activity.<ref name=sarkis2018 /> | |||
| The density of K12-8 b is about {{val|2.67|0.52|0.47|ul=g/cm3}}, intermediate between ] and ] and implying that the planet has a ]-rich envelope.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=1}} The planet may either be rocky with a thick envelope or have a Neptune-like composition,{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=4}} while a pure water planet with a thin atmosphere is less likely.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=5}} | |||
| == Location == | |||
| ] | |||
| K2-18 is in the ] of ], but outside its lion ].<ref name="SIMBAD">{{Cite web | url = http://simbad.u-strasbg.fr/simbad/sim-id?Ident=k2-18 | title = K2-18 -- High proper-motion Star | access-date = 12 September 2019 | work = ]}}</ref> When first discovered, K2-18's distance from Earth was estimated to be {{Convert|34|parsec|ly|sigfig=2|order=flip}}.<ref name="discovery" /> However, more precise data from the '']'' star mapping project has shown K2-18 to be at a distance of {{Convert|38.025|+/-|0.079|parsec|ly|order=flip}}. This improved distance measurement helped to refine the properties of the exoplanetary system.<ref name="Cloutier2019" /> | |||
| === Atmosphere and climate === | |||
| == Physical characteristics == | |||
| K2-18b orbits K2-18 at about {{Convert|0.1429|au|e6km|lk=in|abbr=unit}}, which lies within the calculated habitable zone for the red dwarf, {{Convert|0.12|-|0.25|au|e6km|sigfig=2|abbr=unit}}.<ref name="tsiaras" /> The exoplanet has an ] of about 33 days,<ref name="spitzer" /> which suggests it is ], with the same face to its host star.<ref name='Wall Sep2019' /> The planet's ] is estimated to be around {{Convert|265|+/-|5|K|C F|0|lk=in|abbr=}},<ref name="Cloutier2019" /> due to its ] of approximately 94% of Earth's.<ref name="spitzer" /> K2-18b is estimated to have a radius of {{Val|2.279|0.025|ul=R_Earth}} and a mass of {{Val|8.63|1.35|ul=M_Earth}}, based on analysis using HARPS and CARMENES instruments as well as followup observations from Spitzer.<ref name="Cloutier2019" /><ref name="spitzer" /> It was initially considered a ] on its 2015 discovery,<ref name="discovery" /> but improved data on K2-18b has classified it as a ].<ref name="spitzer" /> A later study from 2019 classified the planet as a sub-Neptune.<ref>{{Cite web|url=https://aasnova.org/2020/01/31/todays-forecast-for-k2-18b-cloudy-with-a-chance-of-rain/|title=Today's Forecast for K2-18b: Cloudy with a Chance of Rain?|website=aasnova.org|date=31 January 2020 |language=en-US|access-date=2020-02-03}}</ref><ref name="benneke2019" /> | |||
| Observations with the ] have found that K12-8 b has an ] consisting of ]. ] makes up between 0.7 and 1.6% of the atmosphere, while ] and ] concentrations appear to be unmeasurably low.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=2}} The atmosphere makes up at most 6.2% of the planet's mass.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=4}} | |||
| <gallery mode="packed" heights="200"> | |||
| File:K2-18 Star System Video.webm|Artist's impression of the K2-18 star system | |||
| File: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 | |||
| </gallery> | |||
| == 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 ] and ].<ref name="Cloutier2017" /> | |||
| There is little evidence for clouds or hazes.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=3}} | |||
| {{External media | |||
| | width = 230px | |||
| | video1 = <br />(video/1:19; 11 September 2019) | |||
| | video2 = <br />] (video/2:03; 11 September 2019) | |||
| }} | |||
| Further studies using the Hubble Space Telescope were performed, corroborating the results of the ''Kepler'' and ''Spitzer'' observations and allowing additional measurements of the ]. Two separate analyses by researchers at ] and ] (UCL) of the Hubble data were published in 2019. Both examined ] 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.<ref name="natgeo" /><ref name="tsiaras" /><ref>{{Cite news|last=Grossman|first=Lisa|url=https://www.sciencenews.org/article/first-known-exoplanet-rain-clouds-water-droplets|title=This may be the first known exoplanet with rain and clouds of water droplets|work=]|date=2019-09-11|access-date=2019-09-11|archive-url=https://web.archive.org/web/20190912065234/https://www.sciencenews.org/article/first-known-exoplanet-rain-clouds-water-droplets|archive-date=12 September 2019|url-status=live}}</ref> The UCL-led study was published on 11 September 2019 in the journal '']''; the study led from the Université de Montréal was posted one day earlier on the ] server ] and later published in ''] Letters''.<ref name='Wall Sep2019'>{{Cite news|url=https://www.space.com/alien-planet-k2-18b-water-vapor-not-earth-twin.html|title=The Water Vapor Find on 'Habitable' Exoplanet K2-18 b Is Exciting — But It's No Earth Twin.|work=]|first=Mike|last=Wall|date=2019-09-11|access-date=2019-09-12|quote=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 ], posted its paper on the online preprint site arXiv.org Tuesday. The study by Benneke et al. has not yet been peer-reviewed.|archive-url=https://web.archive.org/web/20190911230430/https://www.space.com/alien-planet-k2-18b-water-vapor-not-earth-twin.html|archive-date=11 September 2019|url-status=live}}</ref> The UCL-led analysis detected water with a ] of 3.6 ]s, equivalent to a ] of 99.97%.<ref name="tsiaras" /> | |||
| {{wikinews|Astronomers find water vapour in atmosphere of exoplanet K2-18b}} | |||
| This was the first super-Earth exoplanet within a star's habitable zone whose atmosphere was detected,<ref name="tsiaras" /> and the first discovery of water in a habitable-zone exoplanet.<ref name="bbc" /><ref name="natgeo" /> Water had previously been detected in the atmospheres of non-habitable-zone exoplanets such as ], ], ], ], and ].<ref name="NASA-20131203">{{Cite web |title=Hubble Traces Subtle Signals of Water on Hazy Worlds |url=http://www.nasa.gov/content/goddard/hubble-traces-subtle-signals-of-water-on-hazy-worlds/ |date=3 December 2013 |publisher=] |access-date=4 December 2013 |archive-url=https://web.archive.org/web/20131206012837/http://www.nasa.gov/content/goddard/hubble-traces-subtle-signals-of-water-on-hazy-worlds/ |archive-date=6 December 2013 |url-status=live }}</ref><ref name="ApJ-20130910">{{Cite journal | doi = 10.1088/0004-637X/774/2/95|arxiv = 1302.1141 |bibcode = 2013ApJ...774...95D| title = Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope| journal = ]| volume = 774| issue = 2| pages = 95| year = 2013| last1 = Deming | first1 = D. | last2 = Wilkins | first2 = A. | last3 = McCullough | first3 = P. | last4 = Burrows | first4 = A. | last5 = Fortney | first5 = J. J. | last6 = Agol | first6 = E. | last7 = Dobbs-Dixon | first7 = I. | last8 = Madhusudhan | first8 = N. | last9 = Crouzet | first9 = N. | last10 = Desert | first10 = J. M. | last11 = Gilliland | first11 = R. L. | last12 = Haynes | first12 = K. | last13 = Knutson | first13 = H. A. | last14 = Line | first14 = M. | last15 = Magic | first15 = Z. | last16 = Mandell | first16 = A. M. | last17 = Ranjan | first17 = S. | last18 = Charbonneau | first18 = D. | last19 = Clampin | first19 = M. | last20 = Seager | first20 = S. | last21 = Showman | first21 = A. P.|s2cid = 10960488 | display-authors = 1}}</ref><ref name="ApJ-20131203">{{Cite journal | doi = 10.1088/0004-637X/779/2/128|bibcode=2013ApJ...779..128M|arxiv = 1310.2949 | title = Exoplanet Transit Spectroscopy Using WFC3: WASP-12 b, WASP-17 b, and WASP-19 b| journal = ]| volume = 779| issue = 2| pages = 128| year = 2013| last1 = Mandell | first1 = A. M. | last2 = Haynes | first2 = K. | last3 = Sinukoff | first3 = E. | last4 = Madhusudhan | first4 = N. | last5 = Burrows | first5 = A. | last6 = Deming | first6 = D.|s2cid=52997396| display-authors = 1}}</ref> | |||
| There are various possibilities for the temperature and pressure at the atmosphere-ocean boundary.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=4}} | |||
| 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.<ref name="bbc" /> Nevertheless, finding water in a habitable zone exoplanet helps understand how planets are formed.<ref name="bbc" /> A study led by astronomers from the ] 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 ].<ref name="madhusudhan2020" /> K2-18b is now expected to be observed with the ], launched in 2021, and the ], due to launch in 2029. Both will carry instruments designed to determine the composition of ]s.<ref name="natgeo">{{Cite web |last=Greshko |first=Michael |url=https://www.nationalgeographic.com/science/2019/09/first-water-found-in-habitable-exoplanets-atmosphere-hubble-kepler-k2-18b/ |title=Water found on a potentially life-friendly alien planet |work=] |date=11 September 2019 |access-date=11 September 2019 |archive-url=https://web.archive.org/web/20190911201003/https://www.nationalgeographic.com/science/2019/09/first-water-found-in-habitable-exoplanets-atmosphere-hubble-kepler-k2-18b/ |archive-date=11 September 2019 |url-status=live }}</ref> | |||
| K12-8 b is located within the ] of its star, its ] is about {{convert|250|-|300|K}}.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=1}} Whether the planet is actually habitable depends on the nature of the envelope; most scenarios envisage a ] state of the water layer under the envelope at K12-8 b but a liquid water layer is possible.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=6}} | |||
| 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 ]) planets.<ref>{{citation|arxiv=2011.10424|title=Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 B|year=2022|last1=Bézard|first1=Bruno|last2=Charnay|first2=Benjamin|last3=Blain|first3=Doriann|journal=Nature Astronomy |volume=6 |issue=5 |pages=537–540 |doi=10.1038/s41550-022-01678-z |bibcode=2022NatAs...6..537B |s2cid=227118701 }}</ref><ref>{{citation|arxiv=2011.10459|title=1D atmospheric study of the temperate sub-Neptune K2-18b|year=2021|doi=10.1051/0004-6361/202039072|last1=Blain|first1=D.|last2=Charnay|first2=B.|last3=Bézard|first3=B.|journal=Astronomy & Astrophysics|volume=646|pages=A15|bibcode=2021A&A...646A..15B|s2cid=227118713}}</ref> 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.<ref>{{citation|arxiv=2109.14608|year=2021|title=Stellar Surface Inhomogeneities as a Potential Source of the Atmospheric Signal Detected in the K2-18b Transmission Spectrum|doi=10.3847/1538-3881/ac2824|last1=Barclay|first1=Thomas|last2=Kostov|first2=Veselin B.|last3=Colón|first3=Knicole D.|last4=Quintana|first4=Elisa V.|last5=Schlieder|first5=Joshua E.|last6=Louie|first6=Dana R.|last7=Gilbert|first7=Emily A.|last8=Mullally|first8=Susan E.|journal=The Astronomical Journal|volume=162|issue=6|page=300|bibcode=2021AJ....162..300B|s2cid=238215555}}</ref> | |||
| The unusually low ammonia and methane concentrations could be due to ] processes or even due to life.{{sfn|Madhusudhan|Nixon|Welbanks|Piette|2020|p=6}} | |||
| == See also == | |||
| * {{Annotated link|Extraterrestrial liquid water}} | |||
| * {{Annotated link|Habitability of natural satellites#In the Solar System}} | |||
| * {{Annotated link|Habitability of red dwarf systems}} | |||
| * {{Annotated link|List of potentially habitable exoplanets}} | |||
| * {{Annotated link|Planetary habitability}} | |||
| == References == | == References == | ||
| {{Reflist |
{{Reflist}} | ||
| === Sources === | |||
| <ref name="Cloutier2017">{{Cite journal | title=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 | url=https://www.aanda.org/articles/aa/full_html/2017/12/aa31558-17/aa31558-17.html | last1=Cloutier | first1=R. | last2=Astudillo-Defru | first2=N. | last3=Doyon | first3=R. | last4=Bonfils | first4=X. | last5=Almenara | first5=J.-M. | last6=Benneke | first6=B. | last7=Bouchy | first7=F. | last8=Delfosse | first8=X. | last9=Ehrenreich | first9=D. | last10=Forveille | first10=T. | last11=Lovis | first11=C. | last12=Mayor | first12=M. | last13=Menou | first13=K. | last14=Murgas | first14=F. | last15=Pepe | first15=F. | last16=Rowe | first16=J. | last17=Santos | first17=N. C. | last18=Udry | first18=S. | last19=Wünsche | first19=A. | display-authors=1 | journal=] | volume=608 | issue=35 | at=A35 | date=5 December 2017 | arxiv=1707.04292 | bibcode=2017A&A...608A..35C | doi=10.1051/0004-6361/201731558 | s2cid=118955175 | access-date=11 September 2019 | archive-url=https://web.archive.org/web/20180812020209/https://www.aanda.org/articles/aa/full_html/2017/12/aa31558-17/aa31558-17.html | archive-date=12 August 2018 | url-status=live }}</ref> | |||
| {{refbegin}} | |||
| * <ref>{{cite journal |last1=Madhusudhan |first1=Nikku |last2=Nixon |first2=Matthew C. |last3=Welbanks |first3=Luis |last4=Piette |first4=Anjali A. A. |last5=Booth |first5=Richard A. |title=The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b |journal=The Astrophysical Journal Letters |date=February 2020 |volume=891 |issue=1 |pages=L7 |doi=10.3847/2041-8213/ab7229 |url=https://iopscience.iop.org/article/10.3847/2041-8213/ab7229/meta |language=en |issn=2041-8205}}</ref> | |||
| {{refend}} | |||
| <!--- Categories ---> | |||
| <ref name="benneke2019">{{Cite journal |last1=Benneke|first1=Björn|last2=Wong|first2=Ian|last3=Piaulet|first3=Caroline|last4=Knutson|first4=Heather A.|last5=Lothringer|first5=Joshua|last6=Morley|first6=Caroline V.|last7=Crossfield|first7=Ian J. M.|last8=Gao|first8=Peter|last9=Greene|first9=Thomas P.|last10=Dressing|first10=Courtney|last11=Dragomir|first11=Diana|date=December 2019|title=Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b|journal=The Astrophysical Journal|language=en|volume=887|issue=1|pages=L14|doi=10.3847/2041-8213/ab59dc|arxiv=1909.04642|bibcode=2019ApJ...887L..14B|s2cid=209324670|issn=2041-8205}}</ref> | |||
| <ref name="tsiaras">{{Cite journal | first1 = Angelos | last1 = Tsiaras | first2 = Ingo P. | last2 = Waldmann | first3 = Giovanna | last3 = Tinetti | first4 = Jonathan | last4 = Tennyson | first5 = Sergey N. | last5 = Yurchenko | display-authors = 1 | title = Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b | journal = ] | volume = 3 | issue = 12 | pages = 1086–1091 | date = 11 September 2019 | doi = 10.1038/s41550-019-0878-9 | arxiv = 1909.05218 | bibcode = 2019NatAs...3.1086T | s2cid = 202558393 }}</ref> | |||
| <ref name="madhusudhan2020">{{Cite journal | title=The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b | last1=Madhusudhan | first1=Nikku | last2=Nixon | first2=Matthew C. | last3=Welbanks | first3=Luis | last4=Piette | first4=Anjali A. A. | last5=Booth | first5=Richard A. | display-authors=1 | journal=The Astrophysical Journal Letters | volume=891 | issue=1 | at=L7 | date=March 2020 | arxiv=2002.11115 | bibcode=2020ApJ...891L...7M | bibcode-access=free | doi=10.3847/2041-8213/ab7229 | doi-access=free }}</ref> | |||
| }} | |||
| == External links == | |||
| *{{Cite journal|arxiv=1909.04642|last1=Benneke|first1=Björn|title=Water Vapor on the Habitable-Zone Exoplanet K2-18b|last2=Wong|first2=Ian|last3=Piaulet|first3=Caroline|last4=Knutson|first4=Heather A.|last5=Crossfield|first5=Ian J. M.|last6=Lothringer|first6=Joshua|last7=Morley|first7=Caroline V.|last8=Gao|first8=Peter|last9=Greene|first9=Thomas P.|last10=Dressing|first10=Courtney|last11=Dragomir|first11=Diana|last12=Howard|first12=Andrew W.|last13=McCullough|first13=Peter R.|last14=Fortney|first14=Eliza M. -R. Kempton Jonathan J.|last15=Fraine|first15=Jonathan|journal=The Astrophysical Journal |display-authors=1|year=2019|volume=887 |issue=1 |doi=10.3847/2041-8213/ab59dc|bibcode=2019ApJ...887L..14B |s2cid=209324670}} | |||
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Revision as of 08:38, 27 March 2023
K12-8 b new article content ...
Star
A number of planets have been discovered around M dwarf stars, including around LHS 1140, Proxima Centauri and TRAPPIST-1 where they are located within the habitable zone of the stars. The small mass, size and low temperatures of these stars make it easier to characterize these planets.
Physical properties
The density of K12-8 b is about 2.67+0.52
−0.47 g/cm, intermediate between Earth and Neptune and implying that the planet has a hydrogen-rich envelope. The planet may either be rocky with a thick envelope or have a Neptune-like composition, while a pure water planet with a thin atmosphere is less likely.
Atmosphere and climate
Observations with the Hubble Space Telescope have found that K12-8 b has an atmosphere consisting of hydrogen. Water vapour makes up between 0.7 and 1.6% of the atmosphere, while ammonia and methane concentrations appear to be unmeasurably low. The atmosphere makes up at most 6.2% of the planet's mass.
There is little evidence for clouds or hazes.
There are various possibilities for the temperature and pressure at the atmosphere-ocean boundary.
K12-8 b is located within the habitable zone of its star, its equilibrium temperature is about 250–300 K (−23–27 °C; −10–80 °F). Whether the planet is actually habitable depends on the nature of the envelope; most scenarios envisage a supercritical state of the water layer under the envelope at K12-8 b but a liquid water layer is possible.
The unusually low ammonia and methane concentrations could be due to photochemical processes or even due to life.
References
- ^ Madhusudhan et al. 2020, p. 1.
- ^ Madhusudhan et al. 2020, p. 4.
- Madhusudhan et al. 2020, p. 5.
- Madhusudhan et al. 2020, p. 2.
- Madhusudhan et al. 2020, p. 3.
- ^ Madhusudhan et al. 2020, p. 6.
Sources
- Madhusudhan, Nikku; Nixon, Matthew C.; Welbanks, Luis; Piette, Anjali A. A.; Booth, Richard A. (February 2020). "The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b". The Astrophysical Journal Letters. 891 (1): L7. doi:10.3847/2041-8213/ab7229. ISSN 2041-8205.
{{cite journal}}: CS1 maint: unflagged free DOI (link)