The dominant planet formation model: dust grains collide and stick, building pebbles → boulders → planetesimals (~1km). Gravity then dominates, enabling runaway accretion. A body 1000km across grows its own gravitational focusing factor.
At ~2.7 AU, temperatures drop below 150K and water ice condenses. Beyond this "frost line," solid surface density roughly doubles, enabling rapid formation of giant planet cores. Jupiter's core likely formed near the snow line.
If the disk is massive enough (Toomre Q < 1), it fragments directly into gas giants without first forming solid cores. This may form giant planets on wide orbits around massive or rapidly-accreting young stars.
Orbital resonances between Jupiter and Saturn (2:1) destabilized the early solar system ~700 Myr after formation, causing the Late Heavy Bombardment — scattering Uranus and Neptune outward and bombarding the inner planets.