canceled out by Saturn’s torque. Thus, a non-migrating evolution is achieved after the planets lock in resonance.

For completeness and sake of clarity, in the remaining part of this sub-section we elaborate on some considerations already reported in sect. 2.4 of MS01. The principle of Type II migration is that, once a planet opens a gap, it positions itself inside the gap in order to balance the torques received from the inner and the outer parts of the disk. Then, locked into this equilibrium position, it is forced to follow the slow, global viscous evolution of the disk (Lin and Papaloizou, 1986), the latter described by the equations in Lynden-Bell and Pringle (1974). One could expect that the Jupiter-Saturn system should evolve in the same way. The outer migration of the pair of planets should approach Saturn to the outer edge of its gap, until Saturn feels a stronger torque that counterbalances the one received by Jupiter. In this situation the outward migration should stop, and the two planets should start to evolve towards the Sun, together with the disk. This, apparently, does not happen. [For the disk parameters that we explore in this work,]{} Saturn is not massive enough 