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( ${\chi }_{{\mathrm{CH}}_{4}}$ ) = −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 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.

Keywords

• Exoplanet atmospheres
• Exoplanet atmospheric composition
• Exoplanet atmospheric structure