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

Abstract

We find evidence for a strong thermal inversion in the dayside atmosphere of the highly irradiated hot Jupiter WASP-18b (T${}_{eq}=2411K$, $M=10.3M_J$) 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=$283_{-138}^{+395}\times$ 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.

7 pages, 4 figures, 1 table, published in ApJ Letters 11/29

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