We present new results related to the coupled evolution of a two giant planet system embedded in a protoplanetary disk , in which a Saturn mass protoplanet is trapped in an outer mean motion resonance with a Jupiter mass protoplanet .
The gaps opened in the disk by the two planets overlap , therefore the two planet system exchanges angular momentum with the disk at the inner planet ’ s Inner Lindblad Resonances ( ILRs ) and at the outer planet Outer Lindblad Resonances ( OLRs ) .
Since the torques are proportional to the square of the planet masses , and since the inner planet is about three times more massive than the outer one , the ILR torques are favored by a factor \sim 10 with respect to the one planet case .
In the case presented here , this leads to a positive differential Lindblad torque and consequently an outwards migration .
We briefly discuss the long-term behavior of the system , which could account for the high eccentricities of the extra-solar planets with semi-major axis a > 0.2 AU .