Because of its mass (between 7 and 10 Earth masses) and radius (about 2.5 Earth radii), the exoplanet K2-18 b, discovered in 2015 by the Kepler satellite, is considered a "super-Earth" or "mini-Neptune." Moreover, it receives about the same amount of energy from its star, a red dwarf, as the Earth does from the Sun.
By analyzing data obtained by the Hubble Space Telescope recorded during the passage of K2-18 b in front of its star, two teams had announced in 2019 the detection of water vapor in its atmosphere: a first for a planet orbiting in the "habitable zone" of its star. This detection was based on an absorption observed at a wavelength of 1.4 micrometers. These teams had also concluded to the presence of significant quantities of hydrogen and helium and probably of water clouds.
Numerical simulations in support
A LESIA team has produced numerical simulations of the atmosphere of K2-18 b using a coherent physical and chemical model (based on the computer program Exo-REM) and assuming that the exoplanet had a chemical composition close to that of "Neptune". They succeeded in reproducing the Hubble data for K2-18 b, with the best fit being obtained for a proportion of heavy elements (oxygen, carbon, nitrogen, etc.) to hydrogen of about 200 times the solar proportion.
The spectra of the passage of K2-18 b in front of its star, as calculated from this model, show that the absorption observed by Hubble is in fact mostly due to methane, even if it is about half as abundant as water vapor. Methane is a gas that is expected to be abundant in the atmosphere of temperate mini-Neptunes.
More generally, these simulations show that absorption at 1.4 micrometers wavelength is not diagnostic of the presence of water vapor for giant exoplanets with equilibrium temperatures below 600 K.
To differentiate water vapor from methane, it is therefore necessary to observe in parallel at other wavelengths, as the Webb Telescope launched in late 2021 or the European space mission Ariel planned for 2029.
References
B. Bézard, B. Charnay, D. Blain, Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b, 2022, Nature Astronomy.