Goodness of fit of our nominal models, varying only irradiation and metallicity. Left: data from Benneke et al. (2019), including HST, K2, and Spitzer data. Right: data from Tsiaras et al. (2019), including HST, K2, and Spitzer data. The white colour corresponds to the value of χ² of 21.36, indicating a fit of the datasets at the 1σ confidence level. Accordingly, the blue colour indicates overfit and the red colour underfit of the datasets. The solid lines represents the 1 and 2σ confidencelevels of our fits. The dashed and dotted lines represents, respectively, the 1 and 3σ uncertainties on K2-18b irradiation.