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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 bright M dwarf of the spectral class M3. 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 appears to lack star spots, which makes it easier to determine the properties of its planet.

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, although the low luminosity of the stars can make spectroscopic analysis of planets difficult.

Physical properties

K12-8 b has a mass of 8.63±1.35 M_E. 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.

The planet is 2400±600 million years old.

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 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.

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.

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. ^ Benneke et al. 2019, p. 1.
5. Benneke et al. 2019, p. 5.
6. ^ Madhusudhan et al. 2020, p. 1.
7. ^ Charnay et al. 2021, p. 3.
8. ^ Madhusudhan et al. 2020, p. 4.
9. Madhusudhan et al. 2020, p. 5.
10. Benneke et al. 2019, p. 2.
11. Madhusudhan et al. 2020, p. 2.
12. ^ Madhusudhan et al. 2020, p. 3.
13. Benneke et al. 2019, p. 8.
14. Charnay et al. 2021, p. 2.
15. Charnay et al. 2021, p. 1.
16. ^ Madhusudhan et al. 2020, p. 6.
17. Charnay et al. 2021, p. 4.
18. Charnay et al. 2021, pp. 4–7.
19. Charnay et al. 2021, p. 8.
20. Charnay et al. 2021, p. 12.

Sources

• Benneke, Björn; Werner, Michael; Petigura, Erik; Knutson, Heather; Dressing, Courtney; Crossfield, Ian J. M.; Schlieder, Joshua E.; Livingston, John; Beichman, Charles; Christiansen, Jessie; Krick, Jessica; Gorjian, Varoujan; Howard, Andrew W.; Sinukoff, Evan; Ciardi, David R.; Akeson, Rachel L. (January 2017). "SPITZER OBSERVATIONS CONFIRM AND RESCUE THE HABITABLE-ZONE SUPER-EARTH K2-18b FOR FUTURE CHARACTERIZATION". The Astrophysical Journal. 834 (2): 187. doi:10.3847/1538-4357/834/2/187. ISSN 0004-637X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
• Benneke, Björn; Wong, Ian; Piaulet, Caroline; Knutson, Heather A.; Lothringer, Joshua; Morley, Caroline V.; Crossfield, Ian J. M.; Gao, Peter; Greene, Thomas P.; Dressing, Courtney; Dragomir, Diana; Howard, Andrew W.; McCullough, Peter R.; Kempton, Eliza M.-R.; Fortney, Jonathan J.; Fraine, Jonathan (December 2019). "Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b". The Astrophysical Journal Letters. 887 (1): L14. doi:10.3847/2041-8213/ab59dc. ISSN 2041-8205.{{cite journal}}: CS1 maint: unflagged free DOI (link)
• Charnay, B.; Blain, D.; Bézard, B.; Leconte, J.; Turbet, M.; Falco, A. (1 February 2021). "Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b". Astronomy & Astrophysics. 646: A171. doi:10.1051/0004-6361/202039525. ISSN 0004-6361.
• 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)