arXiv: Planet K2-18b is a surfaceless mini-Neptune
Astronomers at NASA’s Ames Research Center have disproved the hypothesis that exoplanet K2-18b is a Hycean, a planet with a hydrogen-rich atmosphere and almost entirely covered in ocean. According to the results of the study, published in an article on the arXiv preprint server, it belongs to mini-Neptunes, which do not have a surface.

K2-18b is located in the habitable zone of a red dwarf star at a distance of about 134 light years from Earth. Its radius reaches 2.6 Earth radii, and its mass is about 8.6 Earth masses. Because the planet is close to its star, the orbital period is only 33 days. But because the star is a dim red dwarf, K2-18b receives about the same amount of energy from its star as Earth does from the Sun.

The planet’s density is between that of Earth and Neptune, meaning it is not composed entirely of rock or gas. It was hypothesized that K2-18b was an oceanic world, which had to be confirmed by studying the composition of the atmosphere.

Observations made with the James Webb Space Telescope found carbon dioxide and methane, indicating that K2-18b may be covered by an ocean of magma. However, in a new paper, astronomers argue that the evidence actually points to the planet being a mini-Neptune.

Thermochemical processes in the deep atmosphere may explain the presence of methane and carbon dioxide detected by JWST, and deep atmospheric kinetics such as upwelling lead to the absence of ammonia and carbon monoxide, the scientists said. Although the Hykean hypothesis remains viable, it requires a biological source of methane, which the authors consider too complex and implausible an explanation.