  We propose that extrasolar asteroid belts can be detected through their
gravitational microlensing signatures. Asteroid belt + star lens systems create
so-called "pseudo-caustics", regions in the source plane where the
magnification exhibits a finite but discontinuous jump. These features allow
such systems to generate distinctive signatures in the microlensing light
curves for a wide range of belt configurations, with source trajectories as far
as tenths of the Einstein ring radius from the centre of the lens. Sample light
curves for a range of asteroid belt parameters are presented. In the near
future, space-based microlensing surveys like WFIRST, which will have the power
to detect percent-level changes in microlensing light curves even with
sub-minute exposure times, may be able to discover extrasolar asteroid belts
with masses of the order of an earth mass.
