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