Joint constraints on the envelope mass fraction and envelope metal content of TOI-270 d. The red (blue) contours spanning the full range of potential envelope metallicities represent the 1σ and 2σ composition constraints obtained with ATHENAIA assuming TOI-270 d’s insolation and a Bond albedo of 0 (0.4). The contours in the top right corner outline the region of the parameter space where fully mixed compositions are unstable (red for A_B = 0, blue for A_B = 0.4, line style encodes planet masses bracketing TOI-270 d). Over the range of atmosphere metal mass fractions allowed by the transmission spectrum, the f_env–Z_env posterior distribution overlaps with the demixing window, where envelope compositional gradients can occur. Vertical gray lines show the locations of various metallicity enrichments relative to solar (labeled). Crosses (slightly offset to the left for clarity) indicate the models where the water condensation curve is crossed: they lie beyond the demixing window and do not affect the conclusions.