Reversing type II migration: resonance trapping of a lighter giant protoplanet

We present a mechanism related to the migration of giant protoplanets embed ded in a protoplanetary disc whereby a giant protoplanet is caught up, befo re having migrated all the way to the central star, by a lighter outer gian t protoplanet. This outer protoplanet may get captured into the 2:3 resonan ce with the more massive one, in which case the gap's that the two planets open in the disc overlap. Two effects arise, namely a squared mass-weighted torque imbalance and an increased mass flow through the overlapping gaps f rom the outer disc to the inner disc, which both play in favour of an outwa rds migration. Indeed, under the conditions presented here, which describe the evolution of a pair of protoplanets respectively Jupiter- and Saturn-si zed, the migration is reversed, while the semimajor axis ratio of the plane ts is constant and the eccentricities are confined to small values by the d isc material. The long-term behaviour of the system is briefly discussed, a nd could account for the high eccentricities observed for the extrasolar pl anets with semimajor axis a > 0.2 au.