Theory of Exozodi Sources and Dust Evolution

Exozodiacal dust disks (exozodis) are populations of warm (~300K) or hot (~1000K) dust, located in or interior to a star's habitable zone, detected around ~25% of main-sequence stars as excess emission over the stellar photosphere at mid- or near-infrared wavelengths. Often too plentiful to be explained by an in-situ planetesimal belt, exozodi dust is usually thought to be transported inwards from further out in the system. There is no consensus on which (if any) of various proposed dynamical models is correct, yet it is vital to understand exozodis given the risk they pose to direct imaging and characterisation of Earth-like planets. This article reviews current theoretical understanding of the origin and evolution of exozodi dust. It also identifies key questions pertinent to the potential for exozodis to impact exoplanet imaging and summarises current understanding of the answer to them informed by exozodi theory. These address how exozodi dust is delivered, its size and spatial distribution, and the effect of its composition on exozodi observability, as well as the connection between hot and warm exozodis. Also addressed are how common different exozodi levels are and how that level can be predicted from system properties, as well as the features that planets impart in dust distributions and how exozodis affect a planet's physical properties and habitability. We conclude that exozodis present both a problem and an opportunity, e.g., by introducing noise that makes planets harder to detect, but also identifying systems in which ingredients conducive to life, like water and volatiles, are delivered to the habitable zone.