Two phase, inward-then-outward migration of Jupiter and Saturn in the gaseous solar nebula

Context. It has recently been shown that the terrestrial planets and asteroid belt can be reproduced if the giant planets underwent an inward-then-outward migration (the “Grand Tack”; Walsh and collaborators). Inward migration occurs when Jupiter opens a gap and type II migrates inward. The planets “tack” and migrate outward when Saturn reaches the gap-opening mass and is caught in the 3:2 resonance with Jupiter.