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

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We find evidence for a strong thermal inversion in the dayside atmosphere of the highly irradiated hot Jupiter WASP-18b (Teq=2411K, M=10.3Mⱼ) 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 1.4μm. However, we infer emission at 4.5μm and absorption at 1.6μm 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⁺³⁹⁵₋₁₃₈× 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 >2σ. Future observations are necessary to confirm the unusual planetary properties implied by these results.