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K12-8 b new article content ...
Star
A number of planets have been discovered around M dwarf stars, including around LHS 1140, Proxima Centauri and TRAPPIST-1 where they are located within the habitable zone of the stars. The small mass, size and low temperatures of these stars make it easier to characterize these planets.
Physical properties
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.
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 for clouds or hazes.
There are various possibilities for the temperature and pressure at the atmosphere-ocean boundary.
K12-8 b is located within the habitable zone of its star, its equilibrium temperature is about 250–300 K (−23–27 °C; −10–80 °F). 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.
The unusually low ammonia and methane concentrations could be due to photochemical processes or even due to life.
References
- ^ Madhusudhan et al. 2020, p. 1.
- ^ Madhusudhan et al. 2020, p. 4.
- Madhusudhan et al. 2020, p. 5.
- Madhusudhan et al. 2020, p. 2.
- Madhusudhan et al. 2020, p. 3.
- ^ Madhusudhan et al. 2020, p. 6.
Sources
- 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.
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