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K12-8 b new article content ...

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.

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.

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.

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). and 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.

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

References

1. ^ Benneke et al. 2019, p. 1.
2. Benneke et al. 2019, p. 5.
3. ^ Madhusudhan et al. 2020, p. 1.
4. ^ Madhusudhan et al. 2020, p. 4.
5. Madhusudhan et al. 2020, p. 5.
6. Benneke et al. 2019, p. 2.
7. Madhusudhan et al. 2020, p. 2.
8. ^ Madhusudhan et al. 2020, p. 3.
9. Benneke et al. 2019, p. 8.
10. ^ Madhusudhan et al. 2020, p. 6.

Sources

1. 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)
2. 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)