An Observational Diagnostic for Distinguishing Between Clouds and Haze in Hot Exoplanet Atmospheres
Kempton et al
The nature of aerosols in hot exoplanet atmospheres is one of the primary vexing questions facing the exoplanet field. The complex chemistry, multiple formation pathways, and lack of easily identifiable spectral features associated with aerosols make it especially challenging to constrain their key properties. We propose a transmission spectroscopy technique to identify the primary aerosol formation mechanism for the most highly irradiated hot Jupiters. The technique is based on the expectation that the two key types of aerosols -- photochemically generated hazes and equilibrium condensate clouds -- are expected to form and persist in different regions of a highly irradiated planet's atmosphere. Haze can only be produced on the permanent daysides of tidally-locked hot Jupiters, and will be carried downwind by atmospheric dynamics to the evening terminator (seen as the trailing limb during transit). Clouds can only form in cooler regions on the night side and morning terminator (seen as the leading limb during transit) of the most highly irradiated giant planets. Because opposite limbs are expected to be impacted by different types of aerosols, ingress and egress spectra, which primarily probe opposing sides of the planet, will reveal the dominant aerosol formation mechanism. In either case, we typically expect the adjacent hemisphere to retain clear skies. Using this diagnostic, we find that observations with JWST and potentially with HST should be able to distinguish between clouds and haze for currently known highly irradiated hot Jupiters.