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

• DOI: 10.48550/arxiv.2310.03245
• Publication Date: 2023-01-01
• Authors: Qiao Xue; Jacob L. Bean; Michael Zhang; Luis Welbanks; J. I. Lunine; Prune C. August
• Keywords: Opacity; Atomic carbon; HITRAN

We present the transmission spectrum of original transiting hot Jupiter HD\,209458b from 2.3 -- 5.1 $\mu$m as observed with NIRCam instrument on James Webb Space Telescope (JWST). Previous studies HD 209458b's atmosphere have given conflicting results abundance H$_2$O and presence carbon- nitrogen-bearing species, which significant ramifications inferences planet's metallicity (M/H) carbon-to-oxygen (C/O) ratio. detect strong features CO$_2$ in JWST spectrum, when interpreted using a retrieval that assumes thermochemical equilibrium fractional grey cloud opacity yields $3^{+4}_{-1}$ $\times$ solar C/O = $0.11^{+0.12}_{-0.06}$. The derived is consistent atmospheric metallicity-planet mass trend gas giants. low ratio suggests this planet has undergone contamination by evaporating planetesimals while migrating inward. are also able to place upper limits abundances CH$_4$, C$_2$H$_2$ HCN log($\chi_{\mathrm{CH}_4}$) -5.6, log($\chi_{\mathrm{C}_2\mathrm{H}_2}$) -5.7, log($\chi_{\mathrm{HCN}}$) -5.1, tension recent claim detection these species ground-based cross-correlation spectroscopy. find weaker feature size than WASP-39b comparing their scale-height-normalized spectra. On other hand, stronger, thus reinforcing inference.