Probing Reflection from Aerosols with the Near-infrared Dayside Spectrum of WASP-80b

The presence of aerosols is intimately linked to the global energy budget and the composition of a planet's atmosphere. Their ability to reflect incoming light prevents energy from being deposited into the atmosphere, and they shape the spectra of exoplanets. We observed five near-infrared secondary eclipses of WASP-80b with the Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope to provide constraints on the presence and properties of atmospheric aerosols. We detect a broadband eclipse depth of 34 ± 10 ppm for WASP-80b. We detect a higher planetary flux than expected from thermal emission alone at 1.6σ, which hints toward the presence of reflecting aerosols on this planet's dayside, indicating a geometric albedo of A_g < 0.33 at 3σ. We paired the WFC3 data with Spitzer data and explored multiple atmospheric models with and without aerosols to interpret this spectrum. Albeit consistent with a clear dayside atmosphere, we found a slight preference for near-solar metallicities and for dayside clouds over hazes. We exclude soot haze formation rates higher than 10^−10.7 g cm⁻²s⁻¹ and tholin formation rates higher than 10^−12.0 g cm⁻²s⁻¹ at 3σ. We applied the same atmospheric models to a previously published WFC3/Spitzer transmission spectrum for this planet and found weak haze formation. A single soot haze formation rate best fits both the dayside and the transmission spectra simultaneously. However, we emphasize that no models provide satisfactory fits in terms of the chi-square of both spectra simultaneously, indicating longitudinal dissimilarity in the atmosphere's aerosol composition.