The classical theory of giant planet formation described in the preceding chapter predicts that massive planets ought to form on approximately circular orbits, with a strong preference for formation in the outer disk at a few AU or beyond. Most currently known extrasolar planets have orbits that are grossly inconsistent with these predictions and, irrespective of the still open question of what the typical planetary system looks like, their existence demands an explanation. Even within the Solar System the existence of a large resonant population of Kuiper Belt Objects, and the time scale problem for the formation of Uranus and Neptune, suggest that the classical theory is at best incomplete.
In this chapter we describe a set of physical mechanisms – gas disk migration, planetesimal scattering, and planet–planet scattering – that promise to reconcile the observed properties of extrasolar planetary systems with theory. The common feature of all of these mechanisms is that they result in energy and angular momentum exchange either among newly formed planets, or between planets and leftover solid or gaseous debris in the system. The exchange of energy and angular momentum drives evolution of the planetary semi-major axis and eccentricity, which can be substantial enough to make the final architecture of the system unrecognizable from its state immediately after planet formation.