Haze optical depth in exoplanet atmospheres varies with rotation rate: Implications for observations

TL;DR

This study investigates how the rotation rate of tidally locked exoplanets influences haze distribution and optical depth at the limb, with implications for observational strategies and characterization of exoplanet atmospheres.

Contribution

It introduces a simplified 3-D circulation model to analyze haze distribution across different rotation regimes, revealing distinct patterns and optimal targets for observation.

Findings

Haze optical depth varies significantly with rotation rate.

Slower rotators (>13 days) have lower haze optical depths at the limb.

Three circulation regimes produce distinct haze distributions.

Abstract

Transmission spectroscopy supports the presence of uncharacterised, light-scattering and -absorbing aerosols in the atmospheres of many exoplanets. The complexity of factors influencing the formation, 3-D transport, radiative impact, and removal of aerosols makes it challenging to match theoretical models to the existing data. Our study simplifies these factors to focus on the interaction between planetary general circulation and haze distribution at the planetary limb. We use an intermediate complexity general circulation model, ExoPlaSim, to simulate idealised organic haze particles as radiatively active tracers in the atmospheres of tidally locked terrestrial planets for 32 rotation rates. We find three distinct 3-D spatial haze distributions, corresponding to three circulation regimes, each with a different haze profile at the limb. All regimes display significant terminator asymmetry. In our parameter space, super-Earth-sized planets with rotation periods greater than 13 days have the lowest haze optical depths at the terminator, supporting the choice of slower rotators as observing targets.