We present a mechanism related to the migration of giant protoplanets embedded in a protoplanetary disc whereby a giant protoplanet is caught up , before having migrated all the way to the central star , by a lighter outer giant protoplanet .
This outer protoplanet may get captured into the 2:3 resonance with the more massive one , in which case the gaps 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 from the outer disc to the inner disc , which both play in favour of an outwards migration .
Indeed under the conditions presented here , which describe the evolution of a pair of protoplanets respectively Jupiter and Saturn sized , the migration is reversed , while the planets semi-major axis ratio is constant and the eccentricities are confined to small values by the disc material .
The long-term behaviour of the system is briefly discussed , and could account for the high eccentricities observed for the extrasolar planets with semi-major axis a > 0.2 AU .