Water- and methane-dominated atmosphere models for LTT 9779 b.Top left: combined transmission spectrum of LTT 9779 b. Overplotted in dark blue is the best-fitting model from our atmosphere retrievals as well as the 1σ and 2σ credible envelopes. Degeneracy between cloud pressure and metallicity (e.g., Benneke & Seager 2013; Benneke et al. 2019) allows for solutions with a subsolar metallicity and deeper clouds (light blue), as well as solutions with extremely high (>1000× solar) metallicity and clouds below the observable photosphere (blue-gray). Both data and models are binned to a constant resolution of R = 50 for visual clarity. Note that the blue-gray model is virtually indistinguishable from the dark blue. Top right: two-dimensional correlation between the atmosphere metallicity and cloud top pressure in our retrieved posterior distribution. The colored points mark the parameter space “locations” of the correspondingly colored atmosphere model. Gray shading denotes regions of parameter space that can be ruled out by population synthesis and interior modeling work (see Section 5). Bottom: same as the top but for the “No H₂O,” CH₄-dominated case. Note that like the dark blue and blue-gray models above, the orange and red models are virtually indistinguishable.