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We present the transmission spectrum of the original transiting hot JupiterHD\,209458b from 2.3 -- 5.1\,$\mu$m as observed with the NIRCam instrument onthe James Webb Space Telescope (JWST). Previous studies of HD\,209458b'satmosphere have given conflicting results on the abundance of H$_2$O and thepresence of carbon- and nitrogen-bearing species, which have significantramifications on the inferences of the planet's metallicity (M/H) andcarbon-to-oxygen (C/O) ratio. We detect strong features of H$_2$O and CO$_2$ inthe JWST transmission spectrum, which when interpreted using a retrieval thatassumes thermochemical equilibrium and fractional grey cloud opacity yields$4^{+5}_{-2}$ $\times$ solar metallicity and C/O = $0.08^{+0.09}_{-0.05}$. Thederived metallicity is consistent with the atmospheric metallicity-planet masstrend observed in solar gas giants. The low C/O ratio suggests that this planethas undergone significant contamination by evaporating planetesimals whilemigrating inward. We are also able to place upper limits on the abundances ofCH$_4$, C$_2$H$_2$ and HCN of log($\chi_{\mathrm{CH}_4}$) = -4.4,log($\chi_{\mathrm{C}_2\mathrm{H}_2}$) = -5.3, and log($\chi_{\mathrm{HCN}}$) =-5.5, which are in tension with the recent claim of a detection of thesespecies using ground-based cross-correlation spectroscopy. We find thatHD\,209458b has a weaker CO$_2$ feature size than WASP-39b when comparing theirscale-height-normalized transmission spectra. On the other hand, the size ofHD\,209458b's H$_2$O feature is stronger, thus reinforcing the low C/Oinference.

JWST transmission spectroscopy of HD 209458b: a super-solar metallicity, a very low C/O, and no evidence of CH4, HCN, or C2H2