Half the known extrasolar planets have orbital eccentricities in excess of
0.3. Such large eccentricities are surprising as it is thought that planets
form in a protoplanetary disk on nearly circular orbits much like the current
states of the solar system planets. Possible explanations for the large
planetary eccentricities include the perturbations that accompany planet-planet
scattering, the tidal interaction between the gas disk and the planets, Kozai's
secular eccentricity cycles, the eccentricity excitation during planetary pair
migration in mean motion resonance, the perturbations by stellar encounters,
stellar-like relaxation that occurs if planets formed through gravitational
instability, and the relative acceleration by the stellar jet system of the
host star with respect to the companion. In this chapter, we comment on the
relevance and characteristics of the various eccentricity origin theories.Comment: 23 pages, 8 figures. Review lecture at the 2006 Aussois Winter School
"Open Problems in Celestial Mechanics". To appear in Lecture Notes in
Physics, Springe
