An extended migration of Neptune into the planetesimal disk before planetary encounters begin can occur if the disk's inner edge was within 2 AU of Neptune's orbit and is most likely if the giant planets began in a 3:2,3:2,2:1,3:2 resonance chain.
An extended migration of Neptune into the planetesimal disk before planetary encounters begin can occur if the disk's inner edge was within 2 AU of Neptune's orbit. This migration begins soon after the protoplanetary disk dissipates, resulting in an early instability, and is most likely if the giant planets began in a 3:2,3:2,2:1,3:2 resonance chain.