Initial state of the five-planet Nice model (Nesvorný 2011; Nesvorný & Morbidelli 2012): Jupiter, Saturn, and three ice giants — including a Neptune-mass fifth giant between Saturn and Uranus — locked in a compact 3:2, 3:2, 2:1, 3:2 mean-motion resonance chain at 5.5–20 AU, plus a planetesimal disk from 24 to 30 AU. Planetesimal-driven migration eventually breaks the resonances, scattering the ice giants outward and ejecting the fifth planet. This model explains the Late Heavy Bombardment, the Kuiper Belt's structure, and Jupiter's Trojan populations. It remains the leading hypothesis for why our outer Solar System looks the way it does today.