A fully-formed Jupiter starts at 3.5 AU, a location expected to be highly favourable for giant planet formation owing to the presence of the so-called snow line. Saturn’s 30 M+ core is initially at 4.5 AU and grows to 60 M+ as Jupiter migrates inward, over 10^5 years. Inward type-I migration of planetary cores is
inhibited in disks with a realistic cooling timescale; thus Saturn’s core remains at 4.5 AU during this phase. Similarly, the cores of Uranus and Neptune begin at 6 and 8 AU and grow from 5 M+, without migrating. Once Saturn reaches 60 M+, its inward migration begins, and is much faster than that of the fully grown Jupiter. Thus, on catching Jupiter, Saturn is trapped in the 2:3 resonance. Here this happens when Jupiter is at 1.5 AU. The direction of migration is then reversed, and the giant planets migrate outward together. In passing, they capture Uranus and Neptune in resonance and these planets are then pushed outwards as well. Saturn, Uranus and Neptune reach their full mass at the end of the migration when Jupiter reaches 5.4 AU. The migration rate decreases exponentially as the gas disk dissipates.