Long range outward migration of giant planets, with application to Fomalhaut b

Recent observations of exoplanets by direct imaging, reveal that giant planets orbit at a few dozens to more than a hundred of AU from their central star. The question of the origin of these planets challenges the standard theories of planet formation. We propose a new way of obtaining such far planets, by outward migration of a pair of planets formed in the 10 AU region. Two giant planets in mean motion resonance in a common gap in the protoplanetary disk migrate outwards, if the inner one is significantly more massive than the outer one. Using hydrodynamical simulations, we show that their semi major axes can increase by almost one order of magnitude. In a flared disk, the pair of planets should reach an asymptotic radius. This mechanism could account for the presence of Fomalhaut b ; then, a second, more massive planet, should be orbiting Fomalhaut at about 75 AU.