not a rule without exceptions. In our Solar System, Jupiter and Saturn should not have migrated substantially, despite they evidently formed in a massive gaseous disk in order to accrete their hydrogen rich atmospheres. In fact, the existence of Uranus and Neptune outside of Saturn’s orbit, and of the Kuiper belt beyond Neptune, constrains the inward migration of Jupiter and Saturn within a few AUs at most (probably much less; see below). In extra-solar systems, with the extension of the timescale of observations, new planets have been found at distances from the parent star comparable to that of Jupiter. Thus, it is important to study non-generic mechanisms that, in some cases, might stop planet migration, or slow it down significantly.

A new tight constrain on the orbits of Jupiter and Saturn at the time of disappearance of the gas disk comes from a recent model developed to explain the origin of the Late Heavy Bombardment (LHB) of the terrestrial planets (Gomes et al., 2005). In addition to the LHB, this model explains the current orbital architecture of the giant planets (orbital separations, eccentricities and mutual inclinations; Tsiganis et al., 2005) the origin and orbital distribution of the Trojans of Jupiter 