Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b

In their Letter, Tsiaras et al. reported the detection of water vapour in atmosphere K2-18 b, an exoplanet 7 to 10 Earth masses located habitable zone M-dwarf star. The is based on absorption feature seen at 1.4 $\mu$m observations transiting with Hubble Space Telescope/Wide Field Camera 3. We have simulated mean temperature structure and composition K2-18b using a radiative-convective equilibrium model we present here corresponding transit spectroscopy calculations. argue that most likely due methane, gas expected be abundant hydrogen-helium cold sub-Neptunes. More generally, show 1.4-$\mu$m spectra not diagnostic presence for sub-Neptunes having effective less than 600 K dominates over methane this wavelength only larger temperatures.