JWST Transmission Spectroscopy of HD 209458b: A Supersolar Metallicity, a Very Low C/O, and No Evidence of CH4, HCN, or C2H2

We present the transmission spectrum of the original transiting hot Jupiter HD 209458b from 2.3 to 5.1 μm as observed with the NIRCam instrument on the James Webb Space Telescope (JWST). Previous studies of HD 209458b's atmosphere have given conflicting results on the abundance of H₂O and the presence of carbon- and nitrogen-bearing species, which have significant ramifications on the inferences of the planet's metallicity (M/H) and carbon-to-oxygen (C/O) ratio. We detect strong features of H₂O and CO₂ in the JWST transmission spectrum, which when interpreted using a retrieval that assumes thermochemical equilibrium and fractional gray cloud opacity yields ${3}_{-1}^{+4}$ × solar metallicity and C/O = ${0.11}_{-0.06}^{+0.12}$ . The derived metallicity is consistent with the atmospheric metallicity–planet mass trend observed in solar gas giants. The low C/O ratio suggests that this planet has undergone significant contamination by evaporating planetesimals while migrating inward. We are also able to place upper limits on the abundances of CH₄, C₂H₂, and HCN of log( ${\chi }_{{\mathrm{CH}}_{4}}$ ) = ‑5.6, log( ${\chi }_{{{\rm{C}}}_{2}{{\rm{H}}}_{2}}$ ) = ‑5.7, and log( ${\chi }_{\mathrm{HCN}}$ ) = ‑5.1, which are in tension with the recent claim of a detection of these species using ground-based cross-correlation spectroscopy. We find that HD 209458b has a weaker CO₂ feature size than WASP-39b when comparing their scale-height-normalized transmission spectra. On the other hand, the size of HD 209458b's H₂O feature is stronger, thus reinforcing the low C/O inference.