The changing carbon-to-oxygen ratio, C/O, in the atmospheres of HD 189733b (top), HD 209458b (bottom) plotted at the substellar point (black solid) and the two terminators (ϕ = 90° – cyan, ϕ = 270° – brown). The initial solar value is C/O = 0.427 (Anders & Grevesse 1989) for both planets. (Note that in these plots the HD 209458b ϕ has a 180° offset compared to HD 189733b.)