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=283−138+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 >2σ. Future
observations are necessary to confirm the unusual planetary properties implied
by these results