Amplitude of two near-infrared methane molecular bands in the transit spectrum of our models of K2-18 b with parametrized hazes as a function of the Bond albedo. As already illustrated in Fig. 8, the amplitude of the methane bands decreases when the albedo of the planet increases. The numbers show the values of n_haze. The shaded area roughly depicts the area of parameter space that could be consistent with both the observations (transit amplitude greater than 100 ppm) and the albedo required to sustain a surface ocean (A ≳ 0.5). The relatively high band amplitude found in the observations rules out haze that are reflective enough to stabilize a liquid surface ocean.