  The expectation that aerodynamic drag will force the solids in a gas-rich
protoplanetary disk to spiral in toward the host star on short timescales is
one of the fundamental problems in planet formation theory. The nominal
efficiency of this radial drift process is in conflict with observations,
suggesting that an empirical calibration of solid transport mechanisms in a
disk is highly desirable. However, the fact that both radial drift and grain
growth produce a similar particle size segregation in a disk (such that larger
particles are preferentially concentrated closer to the star) makes it
difficult to disentangle a clear signature of drift alone. We highlight a new
approach, by showing that radial drift leaves a distinctive "fingerprint" in
the dust surface density profile that is directly accessible to current
observational facilities. Using an analytical framework for dust evolution, we
demonstrate that the combined effects of drift and (viscous) gas drag naturally
produce a sharp outer edge in the dust distribution (or, equivalently, a sharp
decrease in the dust-to-gas mass ratio). This edge feature forms during the
earliest phase in the evolution of disk solids, before grain growth in the
outer disk has made much progress, and is preserved over longer timescales when
both growth and transport effects are more substantial. The key features of
these analytical models are reproduced in detailed numerical simulations, and
are qualitatively consistent with recent millimeter-wave observations that find
gas/dust size discrepancies and steep declines in dust continuum emission in
the outer regions of protoplanetary disks.
