Given the short lifetime of dust grains close to the star, it is likely that exozodiacal dust originates further out in the planetary system. One possible transport mechanism is Poynting-Robertson (P-R) drag, which causes dust grains to lose angular momentum and spiral inwards towards the star. In Rigley & Wyatt (2020), we developed an analytical model which can rapidly be applied across parameter space to predict the levels of exozodiacal dust that should be dragged into the habitable zone of a star from a Kuiper belt analogue. The model was applied to HOSTS survey observations of exozodiacal dust from the Large Binocular Telescope Interferometer (LBTI). The main conclusions were:
- If a system has a cold planetesimal belt, detectable levels of dust should be dragged in.
- This means that non-detections when a cold belt is present implies the presence of intervening planets stopping the dust from migrating in.
- It is also possible for detectable levels of exozodiacal dust to be dragged in from a cold belt which is too faint to detect in the far-infrared, such that absence of an outer debris disc doesn't necessarily predict the absence of an exozodi.
- P-R drag produces relatively low levels of dust compared to previously detected levels, and could only explain a few detections.
Another promising source of exozodiacal dust is exocomets being scattered into the inner planetary system. It has been suggested that the dominant source of zodiacal dust in the solar system is the spontaneous disruption of Jupiter-family comets. In Rigley & Wyatt (2021), we therefore developed a model for the cometary contribution to the zodiacal cloud as a result of spontaneous fragmentation of comets which takes into account the evolution of dust grains due to radiation pressure, mutual collisions, and P-R drag. We showed that:
- Comet fragmentations can produce the correct size and spatial distribution of zodiacal dust, and sustain the zodiacal cloud at its present level.
- The disruption of a large (>100 km) comet can produce spikes in the amount of dust which last for ~1 Myr.
- If comets supply the zodiacal cloud, the level of dust should be stochastic on long timescales due to variations in the sizes and dynamical lifetimes of comets scattered into the inner solar system.
- This means that if exozodis are also cometary in origin, they should be variable and may exhibit a range of brightness levels.