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

Abstract We present the transmission spectrum of the original transiting hot Jupiter HD 209458b from 2.3 to 5.1 μ m 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 H 2 O 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 H 2 O and CO 2 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 CH 4 , C 2 H 2 , and HCN of log( χ CH 4 ) = −5.6, log( χ C 2 H 2 ) = −5.7, and log( χ 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 CO 2 feature size than WASP-39b when comparing their scale-height-normalized transmission spectra. On the other hand, the size of HD 209458b’s H 2 O feature is stronger, thus reinforcing the low C/O inference.