Exploring the limits of the contribution of stellar heterogeneities to the observed transmission spectrum. All panels show our fiducial transmission spectrum at R = 300. Top: the median retrieved model with atmospheric H₂O with informed stellar heterogeneity priors (same model as in Figure 5) explains the observations well. Middle: the median retrieved model where atmospheric H₂O is excluded, and we use the uninformed priors on the stellar heterogeneity parameters (Table 3). This TLSE-only model, even when using realistic stellar properties, cannot explain the observations. Bottom: a forward model that exhibits the maximal effect of TLSE, which requires an unphysically low starspot temperature. Even in this extreme case, the TLSE cannot explain the observations. ✎