Detection of Aerosols at Microbar Pressures in an Exoplanet Atmosphere

TL;DR

This study provides the first direct observational evidence of aerosols at microbar pressures in an exoplanet atmosphere, confirming theoretical predictions and enhancing understanding of haze formation in such environments.

Contribution

It presents the first direct detection of aerosols at microbar pressures in an exoplanet atmosphere using Hubble Space Telescope observations.

Findings

Aerosols cause a wavelength-dependent scattering slope in the spectrum.

The aerosol distribution extends from 40 millibar to microbar pressures.

Results align with haze formation theories based on microphysics.

Abstract

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 $\sim$1 microbar pressures in the atmosphere of the warm sub-Saturn WASP-69b using observations taken with Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) 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 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.