K2-18b

Coordinates: Sky map 11h 30m 14.518s, +07° 35′ 18.257″
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K2-18b
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 m/s2
Temperature265 ± 5 K (−8 ± 5 °C)

K2-18b, also known as EPIC 201912552 b, is an

habitable zone. This means it receives about a similar amount of starlight as the Earth receives from the Sun. Initially discovered with the Kepler space telescope, it was later observed by the James Webb Space Telescope in order to study the planet's atmosphere
.

In 2019 the presence of

gas planet like Uranus or Neptune
than Earth.

Host star

Main article: K2-18

K2-18 is a

K2-18c,[11] which may interact with K2-18b through tides.[b][13]

It is estimated that up to 80% of all

spectroscopic analysis of planets difficult,[14][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.[9]

Physical properties

K2-18b has a radius of 2.610±0.087 

spin-orbit resonance like Mercury is also possible.[16]

The density of K2-18b is about 2.67+0.52
−0.47 

radius valley. Presumably, planets with intermediary radii cannot hold their atmospheres against the tendency of their own energy output and the stellar radiation to drive atmospheric escape.[20] Planets with even smaller radii are known as Super-Earths and those with larger radii as Sub-Neptunes.[21]

The planet may have taken a few million years to form.

high-pressure ice layer on top of a rocky core,[24] which might destabilize the planet's climate by preventing material flows between the core and the ocean.[25]

Possible ocean

At temperatures exceeding the critical point, liquids and gases stop being different phases and there is no longer a separation between an ocean and the atmosphere.[26] It is unclear whether observations imply that a separate liquid ocean exists on K2-18b,[2] and detecting such an ocean is difficult from the outside;[27] its existence cannot be inferred or ruled out solely from the mass and radius of a planet.[28]

The existence of a liquid water ocean is uncertain.

hydrocarbons and ammonia can be lost from an atmosphere to an ocean if it exists; their presence may thus imply the absence of an ocean-atmosphere separation.[32] However, a subsequent work finds that a magma ocean is also capable of dissolving ammonia and explaining the observation results,[33] while another paper suggests that a gas-rich mini-Neptune model is capable of replicating the observed amount of methane and carbon dioxide, while a liquid water ocean model requires the presence of a biosphere in order to produce sufficient amount of methane.[34]

Atmosphere and climate

Observations with the

water vapour is uncertain,[36] with James Webb Space Telescope observations indicating concentrations of less than 0.1%;[37] this may be due to the JWST seeing a dry stratosphere.[31] Ammonia concentrations appear to be unmeasurably low.[e][35] JWST observations indicate that methane and carbon dioxide make up about 1% of the atmosphere.[40] Other carbon oxides were not reported,[41] only an upper limit to their concentrations (a few percent) has been established.[42] The atmosphere makes up at most 6.2% of the planet's mass[18] and its composition probably resembles that of Uranus and Neptune.[43]

There is little evidence of hazes in the atmosphere,

temperature inversion will form at high elevation, yielding a stratosphere.[49]

Evolution

High-energy radiation from the star, such as hard

Lyman alpha radiation emissions during transits of the planet may show the presence of such an exosphere; this discovery requires confirmation.[54]

Alternative scenarios

Detecting atmospheres around planets is difficult, and several reported findings are controversial.[55] Barclay et al. 2021 suggested that the water vapour signal may be due to stellar activity, rather than water in K2-18b'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,[46] and Bézard, Charnay and Blain 2022 proposed that the evidence of water is actually due to methane,[56] although such a scenario is less probable.[57]

Models

Climate models have been used to simulate the climate that K2-18b might have, and an intercomparison of their results for K2-18b is part of the CAMEMBERT project to simulate the climates of sub-Neptune planets.[58] Among the climate modelling efforts made on K2-18b are:

Habitability

Incoming stellar radiation amounts to 1368+114
−107 

equilibrium temperature is about 250 K (−23 °C; −10 °F) to 300 K (27 °C; 80 °F).[14] Whether the planet is actually habitable depends on the nature of the envelope;[30] the deeper layers of the atmosphere may be too hot,[39] while the water-containing layers might[25] or might not have temperatures and pressures suitable for the development of life.[71]

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 K2-18b.[72] According to Madhusudhan et al., several of these markers could be detected by the James Webb Space Telescope after a reasonable number of observations.[73]

Discovery and research history

The planet was discovered in 2015 by the

systematic errors of the observations.[75] Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and the density of the planet being overestimated.[76] The discovery of the spectroscopic signature of water vapour on K2-18b in 2019 was the first discovery of water vapour on an exoplanet that is not a Hot Jupiter[7] and drew a lot of discussion.[27]

K2-18b has been used as a test case for exoplanet studies.[45] The properties of K2-18b have led to the definition of a "hycean planet", a type of planet that has both abundant liquid water and a hydrogen envelope. Planets with such compositions were previously thought to be too hot to be habitable; findings at K2-18b 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.[77] K2-18 b is probably the best known "hycean" planet.[78] Other, non-hycean compositions are possible, both habitable and nonhabitable.[79]

K2-18 b JWST spectra from 2023. Credit: NASA, CSA, ESA, J. Olmstead, N. Madhusudhan

There is some evidence of

methanogenic life[84] or upwards mixing of gases from the deep interior, if the planet is too hot for life.[85]

A

Planetary Society's website in January 2024 featured NASA astrophysicist Knicole Colón describing some of the scientific results from the observations of K2-18 b by JWST. Data from JWST's MIRI instrument is expected to be gathered in January 2024 about which Colón said: "MIRI will be able to see additional features, absorption features from these molecules, and validate again, the presence of what we've seen and even the abundance." Colón also talks about the lack of evidence of water in the atmosphere: "The fact that the JWST data basically didn't find strong evidence of water in the atmosphere, that could indicate a couple things".[86]

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.[9]
  2. ^ Tidal interactions are mutual interactions, mediated by gravity, between astronomical bodies that are in motion with respect of each other.[12]
  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.[17]
  4. ^ A Neptune-like composition implies that apart from water and rock the planet contains substantial amounts of hydrogen and helium.[18]
  5. photochemical processes[30] or the freezing-out of methane.[39]
  6. ^ Hard UV radiation means UV radiation with short wavelengths;[50] shorter wavelengths imply a higher frequency and higher energy per photon.[51]

References

  1. ^ a b Benneke et al. 2019, p. 4.
  2. ^ a b Blain, Charnay & Bézard 2021, p. 2.
  3. ^ a b Barclay et al. 2021, p. 12.
  4. ^ Adams & Engel 2021, p. 163.
  5. ^ a b c d e Benneke et al. 2019, p. 1.
  6. ^ Mendex 2016, p. 5-18.
  7. ^ a b c d Guinan & Engle 2019, p. 189.
  8. ^ a b Benneke et al. 2019, p. 5.
  9. ^ a b Barclay et al. 2021, p. 2.
  10. ^ a b Barclay et al. 2021, p. 10.
  11. ^ a b Blain, Charnay & Bézard 2021, p. 15.
  12. ^ Spohn 2015, p. 2499.
  13. ^ Ferraz-Mello & Gomes 2020, p. 9.
  14. ^ a b c Madhusudhan et al. 2020, p. 1.
  15. ^ Madhusudhan, Piette & Constantinou 2021, p. 13.
  16. ^ Charnay et al. 2021, p. 3.
  17. ^ Raymond 2011, p. 120.
  18. ^ a b c Madhusudhan et al. 2020, p. 4.
  19. ^ Madhusudhan et al. 2020, p. 5.
  20. ^ Benneke et al. 2019, p. 2.
  21. ^ Innes & Pierrehumbert 2022, p. 1.
  22. ^ Blain, Charnay & Bézard 2021, p. 5.
  23. ^ a b Nixon & Madhusudhan 2021, p. 3420.
  24. ^ Nixon & Madhusudhan 2021, pp. 3425–3426.
  25. ^ a b Nixon & Madhusudhan 2021, p. 3429.
  26. ^ Pierrehumbert 2023, p. 2.
  27. ^ a b May & Rauscher 2020, p. 9.
  28. ^ Changeat et al. 2022, p. 399.
  29. ^ Pierrehumbert 2023, p. 6.
  30. ^ a b c Madhusudhan et al. 2020, p. 6.
  31. ^ a b Madhusudhan et al. 2023, p. 7.
  32. ^ Yu et al. 2021, p. 10.
  33. ^ Shorttle et al. 2024, pp. L8.
  34. ^ Wogan et al. 2024, pp. L7.
  35. ^ a b Madhusudhan et al. 2020, p. 2.
  36. ^ Madhusudhan et al. 2023, p. 2.
  37. ^ Madhusudhan et al. 2023, p. 6.
  38. ^ Madhusudhan et al. 2021.
  39. ^ a b Scheucher et al. 2020, p. 16.
  40. ^ Madhusudhan et al. 2023, p. 9.
  41. ^ Bézard, Charnay & Blain 2022, p. 537.
  42. ^ Cubillos & Blecic 2021, p. 2696.
  43. ^ Blain, Charnay & Bézard 2021, p. 18.
  44. ^ Madhusudhan et al. 2020, p. 3.
  45. ^ a b c Hu 2021, p. 5.
  46. ^ a b Blain, Charnay & Bézard 2021, p. 1.
  47. ^ Charnay et al. 2021, p. 2.
  48. ^ Blain, Charnay & Bézard 2021, p. 9.
  49. ^ Hu 2021, p. 20.
  50. ^ Bark et al. 2000, p. 859.
  51. ^ Quintanilla 2015, p. 2651.
  52. ^ a b Santos et al. 2020, p. 1.
  53. ^ Santos et al. 2020, p. 4.
  54. ^ Santos et al. 2020, p. 3.
  55. ^ Changeat et al. 2022, p. 392.
  56. ^ Bézard, Charnay & Blain 2022, p. 538.
  57. ^ Changeat et al. 2022, p. 393.
  58. ^ Christie et al. 2022, p. 6.
  59. ^ Charnay et al. 2021, p. 4.
  60. ^ Charnay et al. 2021, pp. 4–7.
  61. ^ Charnay et al. 2021, p. 8.
  62. ^ Charnay et al. 2021, p. 12.
  63. ^ Innes & Pierrehumbert 2022, p. 5.
  64. ^ Innes & Pierrehumbert 2022, p. 20.
  65. ^ Hu 2021, p. 9.
  66. ^ Hu 2021, p. 16.
  67. ^ Hu 2021, p. 12.
  68. ^ Charnay et al. 2021, p. 1.
  69. ^ Pierrehumbert 2023, p. 1.
  70. ^ Pierrehumbert 2023, p. 7.
  71. ^ Wogan et al. 2024, p. 2.
  72. ^ Madhusudhan, Piette & Constantinou 2021, p. 2.
  73. ^ Madhusudhan, Piette & Constantinou 2021, p. 17.
  74. ^ a b Benneke et al. 2017, p. 1.
  75. ^ Benneke et al. 2017, p. 8.
  76. ^ Benneke et al. 2019, p. 3.
  77. ^ James 2021, p. 7.
  78. ^ Wogan et al. 2024, p. 1.
  79. ^ Madhusudhan et al. 2023, p. 1.
  80. ^ Madhusudhan et al. 2023, p. 11.
  81. ^ Watties 2023.
  82. ^ Burgess 2023.
  83. ^ Wright 2023.
  84. ^ Wogan et al. 2024, p. 4.
  85. ^ Wogan et al. 2024, p. 5.
  86. ^ Colón, Betts & Al-Ahmed 2024.

Sources

External links


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