Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b

We find evidence for a strong thermal inversion in the dayside atmosphere of the highly irradiated hot Jupiter WASP-18b (T-eq = 2411 K, M = 10.3 M-J) based on emission spectroscopy from Hubble Space Telescope secondary eclipse observations and Spitzer eclipse photometry. We demonstrate a lack of water vapor in either absorption or emission at 1.4 mu m. However, we infer emission at 4.5 mu m and absorption at 1.6 mu m that we attribute to CO, as well as a non-detection of all other relevant species (e.g., TiO, VO). The most probable atmospheric retrieval solution indicates a C/O ratio of 1 and a high metallicity (C/H = 283(-138)(+395) solar). The derived composition and T/P profile suggest that WASP-18b is the first example of both a planet with a non-oxide driven thermal inversion and a planet with an atmospheric metallicity inconsistent with that predicted for Jupiter-mass planets at > 2 sigma. Future observations are necessary to confirm the unusual planetary properties implied by these results.