Warm exo-Zodi from cool exo-Kuiper belts: The significance of P-R drag and the inference of intervening planets

Abstract

Poynting-Robertson (P-R) drag has been considered an ineffective mechanism for delivering dust to regions interior to the cool Kuiper belt analogues seen around other Sun-like stars. This conclusion is however based on the very large contrast in dust optical depth between the parent belt and the interior regions that results from the dominance of collisions over drag in systems with detectable cool belts. Here, we show that the levels of habitable zone dust arising from detectable Kuiper belt analogues can be tens to a few hundreds of times greater than the optical depth in the Solar Zodiacal cloud. Dust enhancements of more than a few tens of 'zodi' are expected to hinder future Earth-imaging missions, but relatively few undetectable Kuiper belts result in such levels, particularly around stars older than a few Gyr. Thus, current mid- to far-IR photometric surveys have already identified most of the 20-25 per cent of nearby stars where P-R drag from outer belts could seriously impact Earth-imaging. The Large Binocular Telescope Interferometer should easily detect such warm dust around many nearby stars with outer belts, and will provide insight into currently unclear details of the competition between P-R drag and collisions. Given sufficient confidence in future models, the inevitability of P-R drag means that the non-detection of warm dust where detectable levels were expected could be used to infer additional dust removal process, the most likely being the presence of intervening planets.