Extended Data Fig. 7. Measured transmission spectrum compared to atmospheric forward model grids.

Top: the single best fit for each model grid (shown as solid coloured lines; PICASO 3.0, ATMO, PHOENIX), fits the planet spectrum (Eureka! reduction) with ${\mathrm{\chi }}_{\mathrm{\nu }}^2$ ≤ 1.22. All single best fits prefer at least solar metallicity and substantial cloud cover. Also shown as a grey dashed line is a solar metallicity, stellar C/O ratio atmospheric model, demonstrating the lack of methane absorption seen in the spectrum. Because we can put an upper limit on the CH₄ abundance, the preferred C/O ratio found by the model grids is substellar. Bottom: residuals of each best fit, shown as the model spectrum subtracted from the reduced spectrum and divided by the uncertainty in transit depth. The residuals show wavelength-dependent correlations, the origin of which are unknown and left for a future study.