Fig. 3. Spectra from three independent 1D RCTE models and their residuals, fit to the weighted-average WASP-39b G395H transmission spectrum.

a, Spectra from the three models. b, Their residuals. The models are dominated by absorption from H₂O and CO₂ with a grey-cloud-top pressure corresponding to ≈1 mbar. The models find that the data are best explained by 3–10 times solar metallicity (M/H) and sub-solar to solar C/O (C/O = 0.30–0.46). The extra absorption owing to SO₂, seen in the spectrum around 4.1 μm, is not included in the RCTE model grids and causes a marked impact on the χ²/N (see Fig. 4).