  Planet formation is accompanied by the formation of comet clouds. In systems
where planets migrate on rapid timescales, the diffusive evolution of comet
orbits may stall, resulting in a comet cloud intermediate between a flattened
Kuiper Belt and a spherical Oort cloud. These `failed Oort clouds' may provide
a `smoking gun', indicating that planetary migration has taken place. If some
fraction of the scattered component consists of planetary embryos, it may be
possible to observe transits of such bodies even when the planetary system is
not edge-on to the line of sight.
