(Morbidelli et al., 2005) and the structure of the Kuiper belt (Levison et al., 2007). If true, this model implies that, at the disappearance of the gas, the giant planets and the primordial trans-Neptunian planetesimal disk were originally in a compact configuration. This limits severely the radial migration of the planets during the preceeding phase, dominated by the interactions with the gas. Moreover, the orbits of the planets were quasi-circular, and the ratio of the orbital periods of Saturn and Jupiter was smaller than 2. Therefore, it is important to investigate whether these constraints are consistent with the dynamics of the planets in the gas disk, as far as we understand it.

A pioneer work in this direction has been done by Masset and Snellgrove (2001; MS01 hereafter). In that letter the authors presented the first numerical simulation of the evolution of Jupiter and Saturn in a gas disk. Saturn initially started at twice the heliocentric distance of Jupiter. After a phase of inward runaway migration, Saturn was captured into the 2:3 mean motion resonance with Jupiter. At that point, the planets reversed their migration, moving outward in parallel, while preserving their resonant relationship. This result is interesting for our 