# An Observational Diagnostic for Distinguishing Between Clouds and Haze in Hot Exoplanet Atmospheres

## TL;DR

This paper introduces a transmission spectroscopy method to differentiate between haze and cloud aerosols in hot exoplanet atmospheres by analyzing ingress and egress spectra, aiding understanding of aerosol formation mechanisms.

## Contribution

The study proposes a novel observational diagnostic using ingress and egress spectra to identify aerosol types in highly irradiated hot Jupiters, enhancing atmospheric characterization techniques.

## Key findings

- WASP-121b shows partial aerosol coverage in its spectrum.
- Ingress-egress spectroscopy can distinguish aerosol formation mechanisms.
- JWST and HST can potentially apply this diagnostic to known hot Jupiters.

## Abstract

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 (HIHJs). 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 of HIHJs (seen as the leading limb during transit). 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. We show that the benchmark HIHJ, WASP-121b, has a transmission spectrum consistent with partial aerosol coverage and that ingress-egress spectroscopy would constrain the location and formation mechanism of those aerosols. In general, 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 HIHJs.