K2-18b: Difference between revisions

K2-18b
	Artist's impression of K2-18b (right) orbiting red dwarf K2-18 (left). The exoplanet K2-18c is shown between them.
Discovery
Discovery site	Kepler space telescope
Discovery date	2015
Detection method	Transit
Orbital characteristics
Semi-major axis	0.1429+0.0060; −0.0065 au; 21,380,000 km
Eccentricity	0.20±0.08
Orbital period (sidereal)	32.939623+0.000095; −0.000100 d
Inclination	89.5785°+0.0079°; −0.0088°
Argument of periastron	−0.10+0.81; −0.59 rad (−5.73+46.4; −33.8 °)
Semi-amplitude	3.55+0.57; −0.58 m/s
Star	K2-18
Physical characteristics
Mean radius	2.610±0.087 R🜨
Mass	8.63±1.35 M🜨
Mean density	2.67 g/cm3
Surface gravity	≤1.66 g
Temperature	265 ± 5 K (−8 ± 5 °C)

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Revision as of 16:29, 27 March 2023

K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth.^[4]^[5] 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.^[6] 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.^[7]^[8]^[9]

Star

K12-8 is a M dwarf of the spectral class M3V^[10] in the constellation Leo.^[11] It is colder and smaller than the Sun, having a temperature of 3,457 K (3,184 °C; 5,763 °F) and a radius 45% of the Sun's.^[12] The star is moderately active but whether it has star spots,^[13] which would tend to create false signals when a planet crosses them,^[14] is unclear.^[14]^[13] K12-8 has an additional planet inside of K2-18 b's orbit, K12-8 c.^[15]

It is estimated that 80% of all M dwarf stars have planets in their habitable zones,^[12] including the stars LHS 1140, Proxima Centauri and TRAPPIST-1. The small mass, size and low temperatures of these stars and frequent orbits of the planets make it easier to characterize the planets. On the other hand, the low luminosity of the stars can make spectroscopic analysis of planets difficult^[16]^[12] and the stars are frequently active with flares and inhomogeneous stellar surfaces (faculae and starspots), which can produce erroneous spectral signals when investigating a planet.^[17]

Physical properties

K2-18 b has a mass of 8.63±1.35 M_E. It orbits its star in 33 days,^[12] passing in front of it during its orbit,^[18] and is most likely tidally locked to the star, although a spin-orbit resonance like Mercury is also possible.^[19] There is a third planet around K2-18, named K2-18c,^[20] which may interact with K2-18 b through tides.^[21]

The density of K2-18 b is about 2.67+0.52
−0.47 g/cm³, intermediate between Earth and Neptune and implying that the planet has a hydrogen-rich envelope.^[16] The planet may either be rocky with a thick envelope or have a Neptune-like composition,^[22] while a pure water planet with a thin atmosphere is less likely.^[23] Planets with compositions between that of Earth and Neptune have no analogues in the Solar System and are thus poorly understood. There is evidence that there are well-separated planetary populations with Earth-like and Neptune-like radii, presumably because planets with intermediary radii cannot hold their atmospheres against their own heat's and the stellar radiation's tendency to drive atmospheric escape^[24] and thus end up with a thin atmosphere or none at all.^[25] This distinction is known as the "radius valley" and planets on its smaller side are known as Super-Earths and those larger as Sub-Neptunes.^[26]

The star is 2400±600 million years old^[27] and the planet may have taken a few million years to assemble.^[28] It probably has little internal heat left and tidal heating is unlikely.^[15] Internal heating may increase temperatures at large depths, but is unlikely to significantly impact the surface temperature.^[29] If an ocean exist, it is probably underlaid by a high-pressure ice layer on top of a rocky core,^[30] which might destabilize the planet's climate by preventing core-ocean exchanges.^[31]

Possible ocean

Whether a liquid water ocean on the surface is compatible with observations is unclear,^[32] but improbable.^[33] Discriminating between an atmosphere that rests on an ocean surface or is contiguous with it is difficult from the outside,^[34] but analyses of trace gases such as hydrocarbons and ammonia may provide clues; a surface would be expected to deplete them.^[35] Most scenarios envisage a supercritical state of the water layer under the envelope at K2-18 b but a liquid water layer is possible.^[36] There are various possibilities for the temperature and pressure at the atmosphere-ocean boundary,^[22] information that can not be inferred solely from mass/radius considerations.^[37]

Atmosphere and climate

Observations with the Hubble Space Telescope have found that K2-18 b has an atmosphere consisting of hydrogen. Water vapour makes up between 0.7 and 1.6% of the atmosphere, while ammonia concentrations appear to be unmeasurably low^[a],^[40] and methane may be either present at standard quantities for this type of planet, or strongly depleted.^[41] Carbon oxides were not reported,^[42] their concentrations do not exceed a few percentage points.^[43] The atmosphere makes up at most 6.2% of the planet's mass^[22] and its composition probably resembles that of Uranus and Neptune.^[41]

There is little evidence of hazes in the atmosphere,^[44] while evidence for water clouds, the only species likely to form at K2-18 b,^[45] is conflicting.^[25] If they exist, the clouds are most likely icy but liquid water is possible.^[46] Apart from water ammonium chloride, sodium sulfide, potassium chloride and zinc sulfide can form clouds at the conditions of K2-18 b, depending on the atmosphere's properties.^[47] Most models expect that a temperature inversion will form at high elevations, the atmosphere would contain a stratosphere.^[48]

Formation and evolution

Atmospheres form from the protostellar nebula and can be enriched with heavy elements through erosion of the gas planet's core or through collisions with planetesimals.^[49]

High-energy radiation from the star, such as hard UV radiation and X-rays, is expected to heat the upper atmosphere and fill it with hydrogen formed through the photodissociation of water, thus forming an extended hydrogen-rich exosphere^[50] that can escape from the planet.^[27] The X-ray and UV fluxes that K2-18 b receives from K12-8 are considerably higher than the equivalent fluxes from the Sun;^[27] the hard UV radiation flux provides enough energy to drive this exosphere to escape at a rate of about 350+400
−290 tons per second, too slow to eventually remove the planet's atmosphere during its lifespan.^[51] Observations of decreases of Lyman-alpha radiation emissions during transits of the planet may show the presence of such an exosphere, although the discovery is very tentative.^[52]

Alternative scenarios

Detecting atmospheres around planets is difficult, and several reported findings are controversial.^[53] Barclay et al. 2021 suggested that the water vapour signal may be due stellar activity, rather than the actual presence of water in K2-18 b's atmosphere.^[10] Bézard et al. 2020 proposed that methane may be a more significant component, making up about 3-10% while water may constitute about 5-11% of the atmosphere,^[25] and Bézard, Charnay and Blain 2022 proposed that the evidence of water is actually due to methane,^[54] although such a scenario is less probable.^[55]

Models

Climate models have been used to simulate the climate that K2-18 b might have, and an intercomparison of model results is part of the CAMEMBERT project to simulate the climates of sub-Neptune planets.^[56] Among the climate modelling efforts made on K2-18 b are:

Charnay et al. 2021, assuming that the planet is tidally locked, found an atmosphere with weak temperature gradients and a wind system with descending air on the night side and ascending air on the day side. In the upper atmosphere, radiation absorption by methane produced an inversion layer.^[57] Clouds could only form if the atmosphere had a high metallicity and their properties strongly depended on the size of cloud particles and the composition and circulation of the atmosphere. They formed mainly at the substellar point and the terminator. If there was rainfall, it could not reach the surface; instead it evaporated on the way as virga.^[58] Simulations with a spin-orbit resonance did not substantially alter the cloud distribution.^[59] They also simulated the appearance of the atmosphere during stellar transits.^[60]
Innes and Pierrehumbert 2022 conduced simulations assuming different rotation rates and concluded that except for high rotation rates, there is not a substantial temperature gradient between poles and equator.^[61] They found the existence of jet streams above the equator and at high latitudes, with weaker equatorial jets at the surface.^[62]
Hu 2021 conducted simulations of the planet's chemistry.^[45] They concluded that the photochemistry should not be able to completely remove ammonia from the outer atmosphere^[63] and that carbon oxides and cyanide would form in the middle atmosphere, where they could be detectable.^[64] The model predicts that a sulfur haze layer could form that extends above the water clouds. Such a haze layer would make investigations of the planet's atmosphere much more difficult.^[65]

Habitability

Incoming stellar radiation amounts to 1368+114
−107 W/m², similar to the insolation Earth receives.^[12] K2-18 b is located within or just slightly inside the habitable zone of its star,^[66] - it may be close to the runaway greenhouse threshold^[67] while falling short^[68] - and its equilibrium temperature is about 250–300 K (−23–27 °C; −10–80 °F).^[16] Whether the planet is actually habitable depends on the nature of the envelope;^[36] the deeper layers of the atmosphere may become too hot.^[39] The water layers might reach temperatures and pressures suitable for the development of life.^[31]

Microorganisms from Earth can survive in hydrogen-rich atmospheres, illustrating that hydrogen is no impediment to life. However, a number of biosignature gases used to identify the presence of life are not reliable indicators when found in a hydrogen-rich atmosphere, thus different markers would be needed to identify biological activity at K2-18 b.^[69] Several of these markers could be detected by the James Webb Space Telescope after a reasonable amount of observations.^[70]

Discovery and research history

The planet was discovered in 2015 by the Kepler Space Telescope,^[71] and its existence was later confirmed with the Spitzer Space Telescope and through Doppler velocity techniques.^[50] Analyses of the transits ruled out that they were caused by unseen companion stars,^[71] by multiple planets or systematic errors.^[72] Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and a too high planetary density.^[73] The discovery of the spectroscopic signature of water vapour on K2-18 b in 2019 was the first discovery of water vapour on a planet that is not a Hot Jupiter^[27] and drew a lot of discussion.^[34] In the future, the planet will be observed by the James Webb Space Telescope.^[74]

K2-18 b has been used as a test case for exoplanet studies.^[45] The properties of K2-18 b have led to the definition of a "Hycean planet", a type of planet which combines abundant water with a hydrogen envelope. Planets with such compositions were previously thought to be too hot to be habitable; findings at K2-18 b instead suggest that they might be cold enough to harbour liquid water oceans conducive to life. The strong greenhouse effect of the hydrogen envelope might allow them to remain habitable even at low instellation rates.^[75]

Notes

^ The lack of ammonia and methane in Neptune-like exoplanet atmospheres is known as the "missing methane problem", and is an unresolved mystery As of 2021^[update].^[38] The unusually low ammonia and methane concentrations could be due to life, photochemical processes^[36] or the freezing-out of methane.^[39]

References

^ Cite error: The named reference discovery was invoked but never defined (see the help page).
^ 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. S2CID 73533100.{{cite journal}}: CS1 maint: unflagged free DOI (link)
^ Cite error: The named reference benneke2019 was invoked but never defined (see the help page).
^ ^a ^b ^c 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. S2CID 118828975.
^ University of Cambridge (26 February 2020). "Large exoplanet could have the right conditions for life". Phys.org. Retrieved 26 February 2020.
^ Cohen, Liz (27 August 2021). "Scientists may find life on Earth-like planets covered in oceans within the next few years". CBS News. Retrieved 28 August 2021.
^ Ghosh, Pallab (12 September 2019). "Water found for first time on 'potentially habitable' planet". BBC News. Archived from the original on 12 September 2019. Retrieved 13 September 2019.
^ Cite error: The named reference natgeo was invoked but never defined (see the help page).
^ Cite error: The named reference tsiaras was invoked but never defined (see the help page).
^ ^a ^b Barclay et al. 2021, p. 12.
^ Adams & Engel 2021, p. 163.
^ ^a ^b ^c ^d ^e Benneke et al. 2019, p. 1. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)
^ ^a ^b Benneke et al. 2019, p. 5. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)
^ ^a ^b Barclay et al. 2021, p. 10.
^ ^a ^b Blain, Charnay & Bézard 2021, p. 15.
^ ^a ^b ^c Madhusudhan et al. 2020, p. 1.
^ Barclay et al. 2021, p. 2.
^ Madhusudhan, Piette & Constantinou 2021, p. 13.
^ Charnay et al. 2021, p. 3.
^ Gomes & Ferraz7-Mello 2020, p. 7.
^ Gomes & Ferraz7-Mello 2020, p. 9.
^ ^a ^b ^c Madhusudhan et al. 2020, p. 4.
^ Madhusudhan et al. 2020, p. 5.
^ Benneke et al. 2019, p. 2. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)
^ ^a ^b ^c Blain, Charnay & Bézard 2021, p. 1.
^ Innes & Pierrehumbert 2022, p. 1.
^ ^a ^b ^c ^d Guinan & Engle 2019, p. 189.
^ Blain, Charnay & Bézard 2021, p. 5.
^ Nixon & Madhusudhan 2021, p. 3420.
^ Nixon & Madhusudhan 2021, pp. 3425–3426.
^ ^a ^b Nixon & Madhusudhan 2021, p. 3429.
^ Blain, Charnay & Bézard 2021, p. 2.
^ Pierrehumbert 2023, p. 6.
^ ^a ^b May & Rauscher 2020, p. 9.
^ Yu et al. 2021, p. 10.
^ ^a ^b ^c Madhusudhan et al. 2020, p. 6.
^ Changeat et al. 2022, p. 399.
^ Madhusudhan et al. 2021.
^ ^a ^b Scheucher et al. 2020, p. 16.
^ Madhusudhan et al. 2020, p. 2.
^ ^a ^b Blain, Charnay & Bézard 2021, p. 18.
^ Bézard, Charnay & Blain 2022, p. 537.
^ Cubillos & Blecic 2021, p. 2696.
^ Madhusudhan et al. 2020, p. 3.
^ ^a ^b ^c Hu 2021, p. 5.
^ Charnay et al. 2021, p. 2.
^ Blain, Charnay & Bézard 2021, p. 9.
^ Hu 2021, p. 20.
^ Blain, Charnay & Bézard 2021, p. 6.
^ ^a ^b Santos et al. 2020, p. 1.
^ Santos et al. 2020, p. 4.
^ Santos et al. 2020, p. 3.
^ Changeat et al. 2022, p. 392.
^ Bézard, Charnay & Blain 2022, p. 538.
^ Changeat et al. 2022, p. 393.
^ Christie et al. 2022, p. 6.
^ Charnay et al. 2021, p. 4.
^ Charnay et al. 2021, pp. 4–7.
^ Charnay et al. 2021, p. 8.
^ Charnay et al. 2021, p. 12.
^ Innes & Pierrehumbert 2022, p. 5.
^ Innes & Pierrehumbert 2022, p. 20.
^ Hu 2021, p. 9.
^ Hu 2021, p. 16.
^ Hu 2021, p. 12.
^ Charnay et al. 2021, p. 1.
^ Pierrehumbert 2023, p. 1.
^ Pierrehumbert 2023, p. 7.
^ Madhusudhan, Piette & Constantinou 2021, p. 2.
^ Madhusudhan, Piette & Constantinou 2021, p. 17.
^ ^a ^b Benneke et al. 2017, p. 1.
^ Benneke et al. 2017, p. 8.
^ Benneke et al. 2019, p. 3. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)
^ Christie et al. 2022, p. 3.
^ James 2021, p. 7.

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External links

Benneke, Björn; et al. (2019). "Water Vapor on the Habitable-Zone Exoplanet K2-18b". The Astrophysical Journal. 887 (1). arXiv:1909.04642. Bibcode:2019ApJ...887L..14B. doi:10.3847/2041-8213/ab59dc. S2CID 209324670.{{cite journal}}: CS1 maint: unflagged free DOI (link)
K2-18 b Confirmed Planet Overview Page, NASA Exoplanet Archive

[40] The lack of ammonia and methane in Neptune-like exoplanet atmospheres is known as the "missing methane problem", and is an unresolved mystery As of 2021^[update].^[38] The unusually low ammonia and methane concentrations could be due to life, photochemical processes^[36] or the freezing-out of methane.^[39]

[discovery-1] Cite error: The named reference discovery was invoked but never defined (see the help page).

[sarkis2018-2] 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. S2CID 73533100.{{cite journal}}: CS1 maint: unflagged free DOI (link)

[benneke2019-3] Cite error: The named reference benneke2019 was invoked but never defined (see the help page).

[Cloutier2019-4] 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. S2CID 118828975.

[PHYS-20200226-5] University of Cambridge (26 February 2020). "Large exoplanet could have the right conditions for life". Phys.org. Retrieved 26 February 2020.

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[natgeo-8] Cite error: The named reference natgeo was invoked but never defined (see the help page).

[tsiaras-9] Cite error: The named reference tsiaras was invoked but never defined (see the help page).

[FOOTNOTEBarclayKostovColónQuintana202112-10] Barclay et al. 2021, p. 12.

[FOOTNOTEAdamsEngel2021163-11] Adams & Engel 2021, p. 163.

[FOOTNOTEBennekeWongPiauletKnutson20191-12] Benneke et al. 2019, p. 1. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)

[FOOTNOTEBennekeWongPiauletKnutson20195-13] Benneke et al. 2019, p. 5. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)

[FOOTNOTEBarclayKostovColónQuintana202110-14] Barclay et al. 2021, p. 10.

[FOOTNOTEBlainCharnayBézard202115-15] Blain, Charnay & Bézard 2021, p. 15.

[FOOTNOTEMadhusudhanNixonWelbanksPiette20201-16] Madhusudhan et al. 2020, p. 1.

[FOOTNOTEBarclayKostovColónQuintana20212-17] Barclay et al. 2021, p. 2.

[FOOTNOTEMadhusudhanPietteConstantinou202113-18] Madhusudhan, Piette & Constantinou 2021, p. 13.

[FOOTNOTECharnayBlainBézardLeconte20213-19] Charnay et al. 2021, p. 3.

[FOOTNOTEGomesFerraz7-Mello20207-20] Gomes & Ferraz7-Mello 2020, p. 7.

[FOOTNOTEGomesFerraz7-Mello20209-21] Gomes & Ferraz7-Mello 2020, p. 9.

[FOOTNOTEMadhusudhanNixonWelbanksPiette20204-22] Madhusudhan et al. 2020, p. 4.

[FOOTNOTEMadhusudhanNixonWelbanksPiette20205-23] Madhusudhan et al. 2020, p. 5.

[FOOTNOTEBennekeWongPiauletKnutson20192-24] Benneke et al. 2019, p. 2. sfn error: multiple targets (2×): CITEREFBennekeWongPiauletKnutson2019 (help)

[FOOTNOTEBlainCharnayBézard20211-25] Blain, Charnay & Bézard 2021, p. 1.

[FOOTNOTEInnesPierrehumbert20221-26] Innes & Pierrehumbert 2022, p. 1.

[FOOTNOTEGuinanEngle2019189-27] Guinan & Engle 2019, p. 189.

[FOOTNOTEBlainCharnayBézard20215-28] Blain, Charnay & Bézard 2021, p. 5.

[FOOTNOTENixonMadhusudhan20213420-29] Nixon & Madhusudhan 2021, p. 3420.

[FOOTNOTENixonMadhusudhan20213425–3426-30] Nixon & Madhusudhan 2021, pp. 3425–3426.

[FOOTNOTENixonMadhusudhan20213429-31] Nixon & Madhusudhan 2021, p. 3429.

[FOOTNOTEBlainCharnayBézard20212-32] Blain, Charnay & Bézard 2021, p. 2.

[FOOTNOTEPierrehumbert20236-33] Pierrehumbert 2023, p. 6.

[FOOTNOTEMayRauscher20209-34] May & Rauscher 2020, p. 9.

[FOOTNOTEYuMosesFortneyZhang202110-35] Yu et al. 2021, p. 10.

[FOOTNOTEMadhusudhanNixonWelbanksPiette20206-36] Madhusudhan et al. 2020, p. 6.

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[FOOTNOTEScheucherWunderlichGrenfellGodolt202016-39] Scheucher et al. 2020, p. 16.

[FOOTNOTEMadhusudhanNixonWelbanksPiette20202-41] Madhusudhan et al. 2020, p. 2.

[FOOTNOTEBlainCharnayBézard202118-42] Blain, Charnay & Bézard 2021, p. 18.

[FOOTNOTEBézardCharnayBlain2022537-43] Bézard, Charnay & Blain 2022, p. 537.

[FOOTNOTECubillosBlecic20212696-44] Cubillos & Blecic 2021, p. 2696.

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[FOOTNOTEHu20215-46] Hu 2021, p. 5.

[FOOTNOTECharnayBlainBézardLeconte20212-47] Charnay et al. 2021, p. 2.

[FOOTNOTEBlainCharnayBézard20219-48] Blain, Charnay & Bézard 2021, p. 9.

[FOOTNOTEHu202120-49] Hu 2021, p. 20.

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[FOOTNOTESantosEhrenreichBourrierAstudillo-Defru20201-51] Santos et al. 2020, p. 1.

[FOOTNOTESantosEhrenreichBourrierAstudillo-Defru20204-52] Santos et al. 2020, p. 4.

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[FOOTNOTEInnesPierrehumbert20225-62] Innes & Pierrehumbert 2022, p. 5.

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[FOOTNOTEHu20219-64] Hu 2021, p. 9.

[FOOTNOTEHu202116-65] Hu 2021, p. 16.

[FOOTNOTEHu202112-66] Hu 2021, p. 12.

[FOOTNOTECharnayBlainBézardLeconte20211-67] Charnay et al. 2021, p. 1.

[FOOTNOTEPierrehumbert20231-68] Pierrehumbert 2023, p. 1.

[FOOTNOTEPierrehumbert20237-69] Pierrehumbert 2023, p. 7.

[FOOTNOTEMadhusudhanPietteConstantinou20212-70] Madhusudhan, Piette & Constantinou 2021, p. 2.

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[FOOTNOTEBennekeWernerPetiguraKnutson20171-72] Benneke et al. 2017, p. 1.

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+* {{cite book |last1=Adams |first1=Josephine C. |last2=Engel |first2=Jürgen |title=Life and Its Future |date=2021 |doi=10.1007/978-3-030-59075-8 |isbn=978-3-030-59074-1 |s2cid=238774381 |url=https://link.springer.com/book/10.1007/978-3-030-59075-8 |language=en}}
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+* {{cite journal |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. |title=Stellar Surface Inhomogeneities as a Potential Source of the Atmospheric Signal Detected in the K2-18b Transmission Spectrum |journal=The Astronomical Journal |date=December 2021 |volume=162 |issue=6 |pages=300 |doi=10.3847/1538-3881/ac2824 |s2cid=238215555 |url=https://iopscience.iop.org/article/10.3847/1538-3881/ac2824/meta |language=en |issn=1538-3881}}
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+* {{cite journal |last1=Benneke |first1=Björn |last2=Werner |first2=Michael |last3=Petigura |first3=Erik |last4=Knutson |first4=Heather |last5=Dressing |first5=Courtney |last6=Crossfield |first6=Ian J. M. |last7=Schlieder |first7=Joshua E. |last8=Livingston |first8=John |last9=Beichman |first9=Charles |last10=Christiansen |first10=Jessie |last11=Krick |first11=Jessica |last12=Gorjian |first12=Varoujan |last13=Howard |first13=Andrew W. |last14=Sinukoff |first14=Evan |last15=Ciardi |first15=David R. |last16=Akeson |first16=Rachel L. |title=SPITZER OBSERVATIONS CONFIRM AND RESCUE THE HABITABLE-ZONE SUPER-EARTH K2-18b FOR FUTURE CHARACTERIZATION |journal=The Astrophysical Journal |date=January 2017 |volume=834 |issue=2 |pages=187 |doi=10.3847/1538-4357/834/2/187 |s2cid=12988198 |url=https://iopscience.iop.org/article/10.3847/1538-4357/834/2/187/meta |language=en |issn=0004-637X}}
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+* {{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 |last12=Howard |first12=Andrew W. |last13=McCullough |first13=Peter R. |last14=Kempton |first14=Eliza M.-R. |last15=Fortney |first15=Jonathan J. |last16=Fraine |first16=Jonathan |title=Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b |journal=The Astrophysical Journal Letters |date=December 2019 |volume=887 |issue=1 |pages=L14 |doi=10.3847/2041-8213/ab59dc |s2cid=209324670 |url=https://iopscience.iop.org/article/10.3847/2041-8213/ab59dc/meta |language=en |issn=2041-8205}}
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+* {{cite journal |journal=Monthly Notices of the Royal Astronomical Society |doi=10.1093/mnras/staa1110 |url=https://academic.oup.com/mnras/article-abstract/494/4/5082/5825369|via=[[arXiv]]|title=Tidal evolution of exoplanetary systems hosting potentially habitable exoplanets. The cases of LHS-1140 b-c and K2-18 b-c |year=2020 |last1=Ferraz-Mello |first1=S. |last2=Gomes |first2=G. O. |volume=494 |issue=4 |pages=5082–5090 }}
-* {{cite journal |last1=Guinan |first1=Edward F. |last2=Engle |first2=Scott G. |title=The K2-18b Planetary System: Estimates of the Age and X-UV Irradiances of a Habitable Zone “Wet” Sub-Neptune Planet |journal=Research Notes of the AAS |date=December 2019 |volume=3 |issue=12 |pages=189 |doi=10.3847/2515-5172/ab6086 |url=https://iopscience.iop.org/article/10.3847/2515-5172/ab6086/meta |language=en |issn=2515-5172}}
+* {{cite journal |last1=Guinan |first1=Edward F. |last2=Engle |first2=Scott G. |title=The K2-18b Planetary System: Estimates of the Age and X-UV Irradiances of a Habitable Zone "Wet" Sub-Neptune Planet |journal=Research Notes of the AAS |date=December 2019 |volume=3 |issue=12 |pages=189 |doi=10.3847/2515-5172/ab6086 |s2cid=242743872 |url=https://iopscience.iop.org/article/10.3847/2515-5172/ab6086/meta |language=en |issn=2515-5172}}
-* {{cite journal |last1=Hu |first1=Renyu |title=Photochemistry and Spectral Characterization of Temperate and Gas-rich Exoplanets |journal=The Astrophysical Journal |date=October 2021 |volume=921 |issue=1 |pages=27 |doi=10.3847/1538-4357/ac1789 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ac1789/meta |language=en |issn=0004-637X}}
+* {{cite journal |last1=Hu |first1=Renyu |title=Photochemistry and Spectral Characterization of Temperate and Gas-rich Exoplanets |journal=The Astrophysical Journal |date=October 2021 |volume=921 |issue=1 |pages=27 |doi=10.3847/1538-4357/ac1789 |s2cid=236965630 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ac1789/meta |language=en |issn=0004-637X}}
-* {{cite journal |last1=Innes |first1=Hamish |last2=Pierrehumbert |first2=Raymond T. |title=Atmospheric Dynamics of Temperate Sub-Neptunes. I. Dry Dynamics |journal=The Astrophysical Journal |date=March 2022 |volume=927 |issue=1 |pages=38 |doi=10.3847/1538-4357/ac4887 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ac4887/meta |language=en |issn=0004-637X}}
+* {{cite journal |last1=Innes |first1=Hamish |last2=Pierrehumbert |first2=Raymond T. |title=Atmospheric Dynamics of Temperate Sub-Neptunes. I. Dry Dynamics |journal=The Astrophysical Journal |date=March 2022 |volume=927 |issue=1 |pages=38 |doi=10.3847/1538-4357/ac4887 |s2cid=245353401 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ac4887/meta |language=en |issn=0004-637X}}
-* {{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}}
+* {{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 |s2cid=211505592 |url=https://iopscience.iop.org/article/10.3847/2041-8213/ab7229/meta |language=en |issn=2041-8205}}
-* {{cite journal |last1=Madhusudhan |first1=Nikku |last2=Piette |first2=Anjali A. A. |last3=Constantinou |first3=Savvas |title=Habitability and Biosignatures of Hycean Worlds |journal=The Astrophysical Journal |date=August 2021 |volume=918 |issue=1 |pages=1 |doi=10.3847/1538-4357/abfd9c |url=https://iopscience.iop.org/article/10.3847/1538-4357/abfd9c/meta |language=en |issn=0004-637X}}
+* {{cite journal |last1=Madhusudhan |first1=Nikku |last2=Piette |first2=Anjali A. A. |last3=Constantinou |first3=Savvas |title=Habitability and Biosignatures of Hycean Worlds |journal=The Astrophysical Journal |date=August 2021 |volume=918 |issue=1 |pages=1 |doi=10.3847/1538-4357/abfd9c |s2cid=237290118 |url=https://iopscience.iop.org/article/10.3847/1538-4357/abfd9c/meta |language=en |issn=0004-637X}}
-* {{cite journal |last1=Madhusudhan |first1=Nikku |last2=Constantinou |first2=Savvas |last3=Moses |first3=Julianne I. |last4=Piette |first4=Anjali |last5=Sarkar |first5=Subhajit |title=Chemical Disequilibrium in a Temperate sub-Neptune |journal=JWST Proposal. Cycle 1 |date=1 March 2021 |pages=2722 |url=https://ui.adsabs.harvard.edu/abs/2021jwst.prop.2722M/abstract}}
+* {{cite journal |last1=Madhusudhan |first1=Nikku |last2=Constantinou |first2=Savvas |last3=Moses |first3=Julianne I. |last4=Piette |first4=Anjali |last5=Sarkar |first5=Subhajit |title=Chemical Disequilibrium in a Temperate sub-Neptune |journal=JWST Proposal. Cycle 1 |date=1 March 2021 |pages=2722 |bibcode=2021jwst.prop.2722M |url=https://ui.adsabs.harvard.edu/abs/2021jwst.prop.2722M/abstract}}
-* {{cite journal |last1=May |first1=E. M. |last2=Rauscher |first2=E. |title=From Super-Earths to Mini-Neptunes: Implications of a Surface on Atmospheric Circulation |journal=The Astrophysical Journal |date=April 2020 |volume=893 |issue=2 |pages=161 |doi=10.3847/1538-4357/ab838b |url=https://iopscience.iop.org/article/10.3847/1538-4357/ab838b/meta |language=en |issn=0004-637X}}
+* {{cite journal |last1=May |first1=E. M. |last2=Rauscher |first2=E. |title=From Super-Earths to Mini-Neptunes: Implications of a Surface on Atmospheric Circulation |journal=The Astrophysical Journal |date=April 2020 |volume=893 |issue=2 |pages=161 |doi=10.3847/1538-4357/ab838b |s2cid=214714012 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ab838b/meta |language=en |issn=0004-637X}}
 * {{cite journal |last1=Nixon |first1=Matthew C |last2=Madhusudhan |first2=Nikku |title=How deep is the ocean? Exploring the phase structure of water-rich sub-Neptunes |journal=Monthly Notices of the Royal Astronomical Society |date=17 June 2021 |volume=505 |issue=3 |pages=3414–3432 |doi=10.1093/mnras/stab1500 |url=https://academic.oup.com/mnras/article/505/3/3414/6302002#sec4-3}}
-* {{cite journal |last1=Pierrehumbert |first1=Raymond T. |title=The Runaway Greenhouse on Sub-Neptune Waterworlds |journal=The Astrophysical Journal |date=February 2023 |volume=944 |issue=1 |pages=20 |doi=10.3847/1538-4357/acafdf |url=https://iopscience.iop.org/article/10.3847/1538-4357/acafdf/meta |language=en |issn=0004-637X}}
+* {{cite journal |last1=Pierrehumbert |first1=Raymond T. |title=The Runaway Greenhouse on Sub-Neptune Waterworlds |journal=The Astrophysical Journal |date=February 2023 |volume=944 |issue=1 |pages=20 |doi=10.3847/1538-4357/acafdf |s2cid=254275443 |url=https://iopscience.iop.org/article/10.3847/1538-4357/acafdf/meta |language=en |issn=0004-637X}}
-* {{cite journal |last1=Santos |first1=Leonardo A. dos |last2=Ehrenreich |first2=David |last3=Bourrier |first3=Vincent |last4=Astudillo-Defru |first4=Nicola |last5=Bonfils |first5=Xavier |last6=Forget |first6=François |last7=Lovis |first7=Christophe |last8=Pepe |first8=Francesco |last9=Udry |first9=Stéphane |title=The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b |journal=Astronomy & Astrophysics |date=1 February 2020 |volume=634 |pages=L4 |doi=10.1051/0004-6361/201937327 |url=https://www.aanda.org/articles/aa/abs/2020/02/aa37327-19/aa37327-19.html |language=en |issn=0004-6361}}
+* {{cite journal |last1=Santos |first1=Leonardo A. dos |last2=Ehrenreich |first2=David |last3=Bourrier |first3=Vincent |last4=Astudillo-Defru |first4=Nicola |last5=Bonfils |first5=Xavier |last6=Forget |first6=François |last7=Lovis |first7=Christophe |last8=Pepe |first8=Francesco |last9=Udry |first9=Stéphane |title=The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b |journal=Astronomy & Astrophysics |date=1 February 2020 |volume=634 |pages=L4 |doi=10.1051/0004-6361/201937327 |s2cid=210472526 |url=https://www.aanda.org/articles/aa/abs/2020/02/aa37327-19/aa37327-19.html |language=en |issn=0004-6361}}
-* {{cite journal |last1=Scheucher |first1=Markus |last2=Wunderlich |first2=F. |last3=Grenfell |first3=J. L. |last4=Godolt |first4=M. |last5=Schreier |first5=F. |last6=Kappel |first6=D. |last7=Haus |first7=R. |last8=Herbst |first8=K. |last9=Rauer |first9=H. |title=Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module |journal=The Astrophysical Journal |date=July 2020 |volume=898 |issue=1 |pages=44 |doi=10.3847/1538-4357/ab9084 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ab9084/meta |language=en |issn=0004-637X}}
+* {{cite journal |last1=Scheucher |first1=Markus |last2=Wunderlich |first2=F. |last3=Grenfell |first3=J. L. |last4=Godolt |first4=M. |last5=Schreier |first5=F. |last6=Kappel |first6=D. |last7=Haus |first7=R. |last8=Herbst |first8=K. |last9=Rauer |first9=H. |title=Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module |journal=The Astrophysical Journal |date=July 2020 |volume=898 |issue=1 |pages=44 |doi=10.3847/1538-4357/ab9084 |arxiv=2005.02114 |s2cid=218502474 |url=https://iopscience.iop.org/article/10.3847/1538-4357/ab9084/meta |language=en |issn=0004-637X}}
-* {{cite journal |last1=Yu |first1=Xinting |last2=Moses |first2=Julianne |last3=Fortney |first3=Jonathan |last4=Zhang |first4=Xi |title=How to identify exoplanet surfaces: without seeing them? |date=1 December 2021 |volume=2021 |pages=P42A–05 |url=https://iopscience.iop.org/article/10.3847/1538-4357/abfdc7 |language=en}}
+* {{cite journal |last1=Yu |first1=Xinting |last2=Moses |first2=Julianne |last3=Fortney |first3=Jonathan |last4=Zhang |first4=Xi |title=How to identify exoplanet surfaces: without seeing them? |journal=The Astrophysical Journal |date=1 December 2021 |volume=2021 |issue=1 |pages=P42A–05 |doi=10.3847/1538-4357/abfdc7 |arxiv=2104.09843 |s2cid=233307061 |url=https://iopscience.iop.org/article/10.3847/1538-4357/abfdc7 |language=en}}
 {{refend}}

v t e 2019 in space
« 2018 2020 »
Space probe launches	Beresheet (lunar lander; Feb 2019) LightSail 2 (solar sail demonstration; Jun 2019) Chandrayaan-2 / Vikram / Pragyan (lunar obiter, lander and rover; Jul 2019)
Impact events	Kamchatka meteor (announced) 2019 MO
Selected NEOs	Asteroid close approaches 2018 XB4 2019 AS5 2016 AZ8 66391 Moshup 2019 OK 1620 Geographos 2006 QV89 2100 Ra-Shalom 2019 SU3 2019 TA7 2019 UN13
Exoplanets	AD Leonis b Beta Pictoris c DS Tucanae b Gliese 251 c Gliese 357 d Gliese 588 b c Gliese 687 c Gliese 555 b Gliese 754 b Gliese 784 b HR 5183 b Kepler-47d confirmed K2-288Bb L 1159-16 b c d LP 816-60 b LTT 1445 Ab Luyten's Star d e PDS 70c Proxima Centauri c Struve 2398 Bb Bc Tau Ceti i (hypothesized) Teegarden's Star b c V1298 Tauri b c d e Wolf 359 b c
Discoveries	2019 AQ3 ASASSN-19bt AT2019qiz GRB 190114C EPIC 204376071 GW190412 GW190521g (first-ever light from bh-bh merger) GW190814 (first-ever "mass-gap" collision) J043947.08+163415.7 K2-18b water vapor M87* imaged PSR J0030+0451 mapped PSR J0740+6620 S5-HVS1 2I/Borisov 20 moons of Saturn WD 0145+234 detection of exoasteroid disruption WD J0914+1914
Comets	C/2018 Y1 (Iwamoto) 78P/Gehrels 168P/Hergenrother 163P/NEAT 138P/Shoemaker–Levy 171P/Spahr 289P/Blanpain
Space exploration	New Horizons (encounter with 486958 Arrokoth; Dec 2018 / Jan 2019) Chang'e 4 / Yutu-2 (landing on the far side of the Moon; Jan 2019) Opportunity (end of mission; Aug 2018 / Feb 2019) Hayabusa2 (departure from 162173 Ryugu; Dec 2019)
Outer space portal Category:2018 in outer space — Category:2019 in outer space — Category:2020 in outer space