Thermal Emission Spectra of Ultra Hot Jupiters

Thursday 5 July, 15:40

The hottest hot Jupiters, which have dayside temperatures greater than 2500 K, are compelling targets for thermal emission observations because they are relatively easy to observe, and so their thermal emission spectra should allow precise characterization of their atmospheric compositions and thermal structures. Theory has generally predicted that the spectra of these ultra-hot planets should show emission features because of thermal inversions in their atmospheres. While the spectra of a couple of these planets like WASP-121b and WASP-33b suggest the presence of subtle emission features, most of these planets display blackbody-like spectra. Such unexpected, featureless spectra have in the past prevented the detailed characterization of these planets. We present new observations of the thermal emission spectra of the hot Jupiters HAT-P-7b, WASP-18b, and WASP-103b, all of which appear featureless. We compare the thermal emission spectra of these ultra hot Jupiters and present a new theory to explain their blackbody-like shapes. We find that the lack of significant features in the spectra of these planets can be explained by a combination of H- opacity and thermal dissociation of water in their upper atmospheres. Additionally, we present a new self-consistent retrieval method that allows us to place constraints on the compositions of these planets, even though their spectra display no strong absorption or emission features. Future observations of these planets with the James Webb Space Telescope (JWST) at longer wavelengths may reveal larger molecular features, which will allow direct measurement of their compositions and thermal structures.

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