JWST transmission spectroscopy of HD 209458b: a super-solar 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 -- 5.1 $\mu$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 grey cloud opacity yields $3^{+4}_{-1}$ $\times$ solar metallicity and C/O = $0.11^{+0.12}_{-0.06}$. 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($\chi_{\mathrm{CH}_4}$) = -5.6, log($\chi_{\mathrm{C}_2\mathrm{H}_2}$) = -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 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.