Aggregate Cloud Particle Effects in Exoplanet Atmospheres

TL;DR

This paper presents a simplified framework to understand how aerosol particle morphology, especially fluffy aggregate particles, affects atmospheric opacity and spectral features in exoplanets, emphasizing high-altitude aerosol formation.

Contribution

It introduces a guideline for assessing aerosol effects on exoplanet spectra, focusing on aggregate particle properties and their formation mechanisms at high altitudes.

Findings

Porosity and composition of fluffy particles significantly influence wavelength-independent opacity.

High-altitude aerosols are likely produced locally via UV bombardment, not transported from lower layers.

Aggregate particle effects can explain flat spectra observed in some exoplanet atmospheres.

Abstract

Aerosol opacity has emerged as a critical factor controlling transmission and emission spectra. We provide a simple guideline for the effects of aerosol morphology on opacity and residence time in the atmosphere, as it pertains to transit observations, particularly those with flat spectra due to high altitude aerosols. This framework can be used for understanding complex cloud and haze particle properties before getting into detailed microphysical modeling. We consider high altitude aerosols to be composed of large fluffy particles that can have large residence times in the atmosphere and influence the deposition of stellar flux and/or the emergence of thermal emission in a different way than compact droplet particles, as generally modeled to date for extrasolar planetary atmospheres. We demonstrate the important influence of aggregate particle porosity and composition on the extent of the wavelength independent regime. We also consider how such fluffy particles reach such high altitudes and conclude that the most likely scenario is their local production at high altitudes via UV bombardment and subsequent blanketing of the atmosphere, rather than some mechanism of lofting or transport from the lower atmosphere.