If a system of three or more giant planets form about a star, their orbits may become unstable as they gain mass by accreting gas from the circumstellar disk; subsequent gravitational encounters among these planets can eject one from the system while placing the others into highly eccentric orbits both closer and farther from the star.

--- Evolution of semimajor axis and eccentricity of a trio of Jupiter-mass planets orbiting a solar-mass star. Initial orbits are circular and coplanar, at distances of 5.0, 7.25, and 10 AU from the star. Their orbits become crossing within a few million years. After a series of close encounters the original inner planet is ejected hyperbolically. The planet originally in the outermost orbit is left in a close, eccentric orbit (a=2 AU, e=0.78). The remaining planet has a distant orbit (a=29 AU, e=0.44). (From Weidenschilling and Marzari 1996, Nature, 384, 619)