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K2-18b

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Discovery

The planet was discovered in 2015 by the Kepler Space Telescope. Analyses of the transits ruled out that they were caused by unseen companion stars, by multiple planets or systematic errors. Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and a too high planetary density.

Star

K12-8 is a M dwarf of the spectral class M3V. 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. The star is moderately active but whether it has star spots, which would tend to create false signals when a planet crosses them, is unclear. K12-8 has an additional planet inside of K12-8 b's orbit, K12-8 c.

It is estimated that 80% of all M dwarf stars have planets in their habitable zones, 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 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.

Physical properties

K12-8 b has a mass of 8.63±1.35 ME. It orbits its star in 33 days and is most likely tidally locked to the star, although a spin-orbit resonance like Mercury is also possible.

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. 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 and thus end up with a thin atmosphere or none at all.

The planet is 2400±600 million years old and may have taken a few million years to assemble. It probably has little internal heat left and tidal heating is unlikely.

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 concentrations appear to be unmeasurably low and methane may be either present at standard quantities for this type of planet, or strongly depleted. The atmosphere makes up at most 6.2% of the planet's mass and its composition probably resembles that of Uranus and Neptune. Barclay et al. 2021 suggested that the water vapour signal may be due stellar activity, rather than the actual presence of water in K12-8 b's atmosphere. 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.

There is little evidence of hazes in the atmosphere, while evidence for water clouds is conflicting. If they exist, the clouds are most likely icy but liquid water is possible. Apart from water ammonium chloride, sodium sulfide, potassium chloride and zinc sulfide can form clouds at the conditions of K12-8 b, depending on the atmosphere's properties.

Whether a liquid water ocean on the surface is compatible with observations is unclear. There are various possibilities for the temperature and pressure at the atmosphere-ocean boundary.

K12-8 b is located within or just slightly inside the habitable zone of its star, its equilibrium temperature is about 250–300 K (−23–27 °C; −10–80 °F).

Incoming stellar radiation amounts to 1368+114
−107 W/m, similar to the insolation Earth receives. 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.

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.

Models

Climate models have been used to simulate the climate that K12-8 b might have. 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. 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. Simulations with a spin-orbit resonance did not substantially alter the cloud distribution. They also simulated the appearance of the atmosphere during stellar transits.

Habitability

The unusually low ammonia and methane concentrations could be due to photochemical processes or even due to life.

References

  1. Benneke et al. 2017, p. 1.
  2. Benneke et al. 2017, p. 8.
  3. Benneke et al. 2019, p. 3.
  4. ^ Barclay et al. 2021, p. 12.
  5. ^ Benneke et al. 2019, p. 1.
  6. ^ Benneke et al. 2019, p. 5.
  7. ^ Barclay et al. 2021, p. 10.
  8. ^ Blain, Charnay & Bézard 2021, p. 15.
  9. ^ Madhusudhan et al. 2020, p. 1.
  10. Barclay et al. 2021, p. 2.
  11. ^ Charnay et al. 2021, p. 3.
  12. ^ Madhusudhan et al. 2020, p. 4.
  13. Madhusudhan et al. 2020, p. 5.
  14. Benneke et al. 2019, p. 2.
  15. ^ Blain, Charnay & Bézard 2021, p. 1.
  16. Blain, Charnay & Bézard 2021, p. 5.
  17. Madhusudhan et al. 2020, p. 2.
  18. ^ Blain, Charnay & Bézard 2021, p. 18.
  19. Madhusudhan et al. 2020, p. 3.
  20. Charnay et al. 2021, p. 2.
  21. Blain, Charnay & Bézard 2021, p. 9.
  22. Blain, Charnay & Bézard 2021, p. 2.
  23. Charnay et al. 2021, p. 1.
  24. ^ Madhusudhan et al. 2020, p. 6.
  25. Blain, Charnay & Bézard 2021, p. 6.
  26. Charnay et al. 2021, p. 4.
  27. Charnay et al. 2021, pp. 4–7.
  28. Charnay et al. 2021, p. 8.
  29. Charnay et al. 2021, p. 12.

Sources