Detection of Aerosols at Microbar Pressures in an Exoplanet Atmosphere

The formation of hazes at microbar pressures has been explored by theoretical models of exoplanet atmospheres to explain Rayleigh scattering and/or featureless transmission spectra; however observational evidence of aerosols in the low-pressure formation environments has proved elusive. Here, we show direct evidence of aerosols existing at similar to 1 microbar pressures in the atmosphere of the warm sub-Saturn WASP-69b using observations taken with the Space Telescope Imaging Spectrograph and Wide Field Camera 3 instruments on the Hubble Space Telescope. The transmission spectrum shows a wavelength-dependent slope induced by aerosol scattering that covers 11 scale heights of spectral modulation. Drawing on the extensive studies of haze in our solar system, we model the transmission spectrum based on a scaled version of Jupiter's haze-density profile to show that the WASP-69b transmission spectrum can be produced by scattering from an approximately constant density of particles extending throughout the atmospheric column from 40 millibar to microbar pressures. These results are consistent with theoretical expectations based on microphysics of the aerosol particles that have suggested haze can exist at microbar pressures in exoplanet atmospheres.

Automated summary

Observational evidence of aerosols existing in the atmosphere of the warm sub-Saturn WASP-69b at around 1 microbar pressures has been presented, indicating the potential presence of haze in exoplanet atmospheres. The transmission spectrum shows a wavelength-dependent slope induced by aerosol scattering covering 11 scale heights of spectral modulation, with modeling based on Jupiter's haze-density profile supporting these findings. These results are consistent with theoretical expectations suggesting haze can exist at microbar pressures in exoplanet atmospheres, drawing on extensive studies of haze in our solar system.