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

TL;DR

This study uses JWST transmission spectroscopy to analyze HD 209458b's atmosphere, revealing super-solar metallicity, very low C/O ratio, and no evidence of certain carbon- and nitrogen-bearing molecules, refining our understanding of its composition.

Contribution

First JWST transmission spectrum of HD 209458b providing new constraints on its atmospheric metallicity and C/O ratio, and challenging previous ground-based claims of certain molecules.

Findings

Metallicity is about 3 times solar, consistent with planetary mass trends.

C/O ratio is approximately 0.11, indicating contamination by planetesimals.

No detection of CH4, C2H2, or HCN at the sensitivity of JWST.

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.