  By logging encounters between planetesimals and planets we compute the
distribution of encounters in a numerically integrated two planet system that
is migrating due to interactions with an exterior planetesimal belt. Capture of
an irregular satellite in orbit about a planet through an exchange reaction
with a binary planetesimal is only likely when the binary planetesimal
undergoes a slow and close encounter with the planet. In our simulations we
find that close and slow encounters between planetesimals and a planet
primarily occur with the outermost and not innermost planet. Taking care to
consider where a planet orbit crossing binary planetesimal would first be
tidally disrupted, we estimate the probability of both tidal disruption and
irregular satellite capture. We estimate that the probability that the
secondary of a binary planetesimal is captured and becomes an irregular
satellite about a Neptune mass outer planet is about 1/100 for binaries with
masses and separations similar to transneptunian planetesimal binaries. If
young exoplanetary debris disks host a binary planetesimal population then
outwards migrating outer planets should host captured irregular satellite
populations. We discuss interpretation of emission associated with the
exoplanet Fomalhaut b in terms of collisional evolution of a captured irregular
satellite population that is replenished due to planetary migration.
