K2-18b

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. 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. It has a 33-day orbit within the star's habitable zone, meaning that it receives about the same amount of starlight as the Earth receives from the Sun and could have similar conditions, which allow the existence of liquid water.

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 siteKepler space telescope
Discovery date2015
Transit
Orbital characteristics[1]
0.15910+0.00046
−0.00047 au
21,380,000 km
Eccentricity0.09+0.12
−0.09[2]
32.940045±0.000100 d
StarK2-18
Physical characteristics
Mean radius
2.610±0.087 R🜨
Mass8.63±1.35 M🜨
Mean density
2.67+0.52
−0.47 g/cm3
12.43+2.17
−2.07 g
Temperature265 ± 5 K (−8 ± 5 °C)

    In 2019 the presence of water vapour in K2-18b's atmosphere was discovered, drawing attention to this system. Its atmosphere may have an unexpectedly small concentration of methane, although observation uncertainties preclude a definitive determination. K2-18b has been studied as a potential habitable word that, temperature aside, resembles more a gas planet like Uranus or Neptune than Earth.

    Star
    Main article: K2-18

    K2-18 is a M dwarf of the spectral class M3V[3] in the constellation Leo,[4] 38.025 ± 0.079 parsecs (124.02 ± 0.26 ly) away from the Sun.[1] The star 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,[5] and is not visible to the naked eye.[6] The star displays moderate stellar activity but whether it has star spots,[7] which would tend to create false signals[lower-alpha 1] when a planet crosses them,[9] is unclear.[9][7] K2-18 has an additional planet inside of K2-18 b's orbit, K2-18c,[10] which may interact with K2-18 b through tides[lower-alpha 2].[12]

    It is estimated that 80% of all M dwarf stars have planets in their habitable zones,[5] 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,[13][5] 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.[8]

    Physical properties

    K2-18 b has a mass of 8.63±1.35 MEarth. It orbits its star in 33 days,[5] from Earth it can be seen passing in front of the star.[14] The planet is most likely tidally locked to the star, although a spin-orbit resonance like Mercury is also possible.[15]

    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[lower-alpha 3].[13] The planet may either be rocky with a thick envelope or have a Neptune-like composition[lower-alpha 4],[17] while a pure water planet with a thin atmosphere is less likely.[18] Planets with compositions intermediate between that of Earth and Neptune have no analogues in the Solar System and are thus poorly understood. There is evidence that there are distinct planetary populations with Earth-like and Neptune-like radii, presumably because planets with intermediary radii cannot hold their atmospheres against the tendency of their own energy output and of the stellar radiation to drive atmospheric escape[19] and thus end up with a thin atmosphere or none at all.[20] This distinction between planet populations is known as the radius valley and planets on its smaller side are known as Super-Earths and those larger as Sub-Neptunes.[21]

    The star is 2400±600 million years old[22] and the planet may have taken a few million years to assemble.[23] It probably has little internal heat left and tidal heating is unlikely.[10] If it exists, internal heating may increase temperatures at large depths, but is unlikely to significantly affect the surface temperature.[24] If an ocean exists, it is probably underlaid by a high-pressure ice layer on top of a rocky core,[25] which might destabilize the planet's climate by preventing material flows between the core and the ocean.[26]

    Possible ocean

    Whether a liquid water ocean on the surface is compatible with observations is unclear,[2] but improbable.[27] Discriminating between an atmosphere that rests on an ocean surface or is contiguous with it is difficult from the outside,[28] but analyses of trace gases such as hydrocarbons and ammonia may provide clues; a surface would be expected to deplete them.[29] Most scenarios envisage a supercritical state of the water layer under the envelope at K2-18 b but a liquid water layer is possible.[30] There are various possibilities for the temperature and pressure at the atmosphere-ocean boundary,[17] information that can not be inferred solely from mass/radius considerations.[31]

    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[lower-alpha 5],[34] and methane may be either present at standard quantities for this type of planet, or strongly depleted.[35] Carbon oxides were not reported,[36] their concentrations do not exceed a few percentage points.[37] The atmosphere makes up at most 6.2% of the planet's mass[17] and its composition probably resembles that of Uranus and Neptune.[35]

    There is little evidence of hazes in the atmosphere,[38] while evidence for water clouds, the only species likely to form at K2-18 b,[39] is conflicting.[20] If they exist, the clouds are most likely icy but liquid water is possible.[40] 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.[41] Most models expect that a temperature inversion will form at high elevations, the atmosphere would contain a stratosphere.[42]

    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.[43]

    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[44] that can escape from the planet.[22] The X-ray and UV fluxes that K2-18 b receives from K2-18 are considerably higher than the equivalent fluxes from the Sun;[22] 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.[45] 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.[46]

    Alternative scenarios

    Detecting atmospheres around planets is difficult, and several reported findings are controversial.[47] 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.[3] 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,[20] and Bézard, Charnay and Blain 2022 proposed that the evidence of water is actually due to methane,[48] although such a scenario is less probable.[49]

    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.[50] 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.[51] 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.[52] Simulations with a spin-orbit resonance did not substantially alter the cloud distribution.[53] They also simulated the appearance of the atmosphere during stellar transits.[54]
    • 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.[55] They found the existence of jet streams above the equator and at high latitudes, with weaker equatorial jets at the surface.[56]
    • Hu 2021 conducted simulations of the planet's chemistry.[39] They concluded that the photochemistry should not be able to completely remove ammonia from the outer atmosphere[57] and that carbon oxides and cyanide would form in the middle atmosphere, where they could be detectable.[58] 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.[59]

    Habitability

    Incoming stellar radiation amounts to 1368+114
    −107     W/m2    , similar to the insolation Earth receives.[5] K2-18 b is located within or just slightly inside the habitable zone of its star,[60] - it may be close to the runaway greenhouse threshold[61] while falling short[62] - and its equilibrium temperature is about 250–300 K (−23–27 °C; −10–80 °F).[13] Whether the planet is actually habitable depends on the nature of the envelope;[30] the deeper layers of the atmosphere may become too hot.[33] The water layers might reach temperatures and pressures suitable for the development of life.[26]

    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.[63] Several of these markers could be detected by the James Webb Space Telescope after a reasonable amount of observations.[64]

    Discovery and research history

    The planet was discovered in 2015 by the Kepler Space Telescope,[65] and its existence was later confirmed with the Spitzer Space Telescope and through Doppler velocity techniques.[44] Analyses of the transits ruled out that they were caused by unseen companion stars,[65] by multiple planets or systematic errors.[66] Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and a too high planetary density.[67] 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[22] and drew a lot of discussion.[28] In the future, the planet will be observed by the James Webb Space Telescope.[68]

    K2-18 b has been used as a test case for exoplanet studies.[39] 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.[69]

    See also

    Notes
    1. Observations of transiting planets rely on comparing the appearance of the planet with the appearance of the star's surface that is not covered with the planet, so variations in the star's appearance can be confused with the effects of the planet.[8]
    2. Tidal interactions are mutual interactions, mediated by gravity, between astronomical bodies that are in motion with respect of each other.[11]
    3. An envelope is an atmosphere that originated together with the planet itself from a protoplanetary disk. In gas giants, atmospheres make up the bulk of the planet's mass.[16]
    4. A Neptune-like composition implies that apart from water and rock the planet contains substantial amounts of hydrogen and helium.[17]
    5. 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.[32] The unusually low ammonia and methane concentrations could be due to life, photochemical processes[30] or the freezing-out of methane.[33]

    References
    1. Benneke et al. 2019, p. 4.
    2. Blain, Charnay & Bézard 2021, p. 2.
    3. Barclay et al. 2021, p. 12.
    4. Adams & Engel 2021, p. 163.
    5. Benneke et al. 2019, p. 1.
    6. Mendex 2016, p. 5-18.
    7. Benneke et al. 2019, p. 5.
    8. Barclay et al. 2021, p. 2.
    9. Barclay et al. 2021, p. 10.
    10. Blain, Charnay & Bézard 2021, p. 15.
    11. Spohn 2015, p. 2499.
    12. Ferraz-Mello & Gomes 2020, p. 9.
    13. Madhusudhan et al. 2020, p. 1.
    14. Madhusudhan, Piette & Constantinou 2021, p. 13.
    15. Charnay et al. 2021, p. 3.
    16. Raymond 2011, p. 120.
    17. Madhusudhan et al. 2020, p. 4.
    18. Madhusudhan et al. 2020, p. 5.
    19. Benneke et al. 2019, p. 2.
    20. Blain, Charnay & Bézard 2021, p. 1.
    21. Innes & Pierrehumbert 2022, p. 1.
    22. Guinan & Engle 2019, p. 189.
    23. Blain, Charnay & Bézard 2021, p. 5.
    24. Nixon & Madhusudhan 2021, p. 3420.
    25. Nixon & Madhusudhan 2021, pp. 3425–3426.
    26. Nixon & Madhusudhan 2021, p. 3429.
    27. Pierrehumbert 2023, p. 6.
    28. May & Rauscher 2020, p. 9.
    29. Yu et al. 2021, p. 10.
    30. Madhusudhan et al. 2020, p. 6.
    31. Changeat et al. 2022, p. 399.
    32. Madhusudhan et al. 2021.
    33. Scheucher et al. 2020, p. 16.
    34. Madhusudhan et al. 2020, p. 2.
    35. Blain, Charnay & Bézard 2021, p. 18.
    36. Bézard, Charnay & Blain 2022, p. 537.
    37. Cubillos & Blecic 2021, p. 2696.
    38. Madhusudhan et al. 2020, p. 3.
    39. Hu 2021, p. 5.
    40. Charnay et al. 2021, p. 2.
    41. Blain, Charnay & Bézard 2021, p. 9.
    42. Hu 2021, p. 20.
    43. Blain, Charnay & Bézard 2021, p. 6.
    44. Santos et al. 2020, p. 1.
    45. Santos et al. 2020, p. 4.
    46. Santos et al. 2020, p. 3.
    47. Changeat et al. 2022, p. 392.
    48. Bézard, Charnay & Blain 2022, p. 538.
    49. Changeat et al. 2022, p. 393.
    50. Christie et al. 2022, p. 6.
    51. Charnay et al. 2021, p. 4.
    52. Charnay et al. 2021, pp. 4–7.
    53. Charnay et al. 2021, p. 8.
    54. Charnay et al. 2021, p. 12.
    55. Innes & Pierrehumbert 2022, p. 5.
    56. Innes & Pierrehumbert 2022, p. 20.
    57. Hu 2021, p. 9.
    58. Hu 2021, p. 16.
    59. Hu 2021, p. 12.
    60. Charnay et al. 2021, p. 1.
    61. Pierrehumbert 2023, p. 1.
    62. Pierrehumbert 2023, p. 7.
    63. Madhusudhan, Piette & Constantinou 2021, p. 2.
    64. Madhusudhan, Piette & Constantinou 2021, p. 17.
    65. Benneke et al. 2017, p. 1.
    66. Benneke et al. 2017, p. 8.
    67. Benneke et al. 2019, p. 3.
    68. Christie et al. 2022, p. 3.
    69. James 2021, p. 7.

    Sources

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