Outward migration of extrasolar planets to large orbital radii

Observations of structure in circumstellar debris discs provide circumstantial evidence for the presence of massive planets at large (several tens of astronomical units) orbital radii, where the time-scale 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 asimilar to10 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. Secondly, 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.