Black circles and error bars indicate the previous HST/WFC3 observations (de Wit et al. 2018) for TRAPPIST-1 planets d, e, f, and g. The blue solid lines indicate our baseline aerosol-free solar composition case. The rest are models of a hydrogen-rich atmosphere containing Rayleigh scattering haze, with mixing ratios for water, carbon dioxide, nitrogen, and methane at 1%. Green dotted lines indicate a zero magnitude haze scattering cross section; purple dashed lines indicate that the haze cross section was increased to give 1σ agreement with the HST data; orange dash–dot lines display haze cross sections increased to give 3σ agreement with the HST data. Only planet d results in the clear exclusion of a haze-free atmosphere.