A planetary system may undergo significant radial rearrangement during the early part of its lifetime. Planet migration can come about through interaction with the surrounding planetesimal disk and the gas disk—while the latter is still present—as well as through planet-planet interactions. We review the major proposed migration mechanisms in the context of the planet formation process, in our Solar System as well as in others.

Introduction

The word planet is derived from the Greek word “planetes,” meaning wandering star. Geocentric views of the Universe held sway until the Middle Ages, when Copernicus and Kepler developed a better phenomenological explanation of planetary wanderings, which with small modifications has withstood the test of time. Kepler's first law of planetary motion states that planets travel along elliptical paths with one focus at the Sun. Thus, although planets wander about the sky, in this model their orbits remain fixed and they do not migrate. In his physical model of the Solar System, Newton theorized that planets gradually altered one another's orbits, and he felt compelled to hypothesize occasional divine intervention to keep planetary trajectories well-behaved over long periods of time. In the early 1800s, Poisson pointed out that planetary-type perturbations cannot produce secular changes in orbital elements to second order in the mass ratio of the planets to the Sun, but Poincare's work towards the end of the 19th century suggests that the Solar System may be chaotic.