purposes in two respects. First, it shows that a two-planet system in some configurations can avoid migration toward the central star. Second, the stable relative configuration achieved by Saturn and Jupiter is characterized by a ratio of orbital periods smaller than 2, as required by the LHB model discussed above.

Therefore, we think that it is timely to investigate more in detail the mechanism unveiled by MS01, in particular exploring a wider range of parameters. In fact, there are a few open issues on the validity of the mechanism and its consistency with the solar system structure that we need to address:

-   it is unclear how the mechanism depends on the adopted initial conditions. If the initial, inward runaway migration of Saturn is a crucial aspect, then the overall result might depend on the numerical resolution of the disk and on its initial state (namely, whether the planets are dropped into a virgin disk, or they are allowed to sculpt the disk for some time before they are let free to migrate). In fact, some researchers could not successfully reproduce the MS01 simulation (Kley, private communication), possibly because of these issues. Moreover, a wide range inward migration of Saturn 