  Observations of structure in circumstellar debris discs provide
circumstantial evidence for the presence of massive planets at large (several
tens of au) orbital radii, where the timescale for planet formation via core
accretion is prohibitively long. Here, we investigate whether a population of
distant planets can be produced via outward migration subsequent to formation
in the inner disc. Two possibilities for significant outward migration are
identified. First, cores that form early at radii of around 10 au can be
carried to larger radii via gravitational interaction with the gaseous disc.
This process is efficient if there is strong mass loss from the disc - either
within a cluster or due to photoevaporation from a star more massive than the
Sun - but does not require the extremely destructive environment found, for
example, in the core of the Orion Nebula. We find that, depending upon the disc
model, gas disc migration can yield massive planets (several Jupiter masses) at
radii of around 20-50 au. Second, interactions within multiple planet systems
can drive the outer planet into a large, normally highly eccentric orbit. A
series of scattering experiments suggests that this process is most efficient
for lower mass planets within systems of unequal mass ratio. This mechanism is
a good candidate for explaining the origin of relatively low mass giant planets
in eccentric orbits at large radii.
