  The known extrasolar planets exhibit many interesting and surprising
features--extremely short-period orbits, high-eccentricity orbits, mean-motion
and secular resonances, etc.--and have dramatically expanded our appreciation
of the diversity of possible planetary systems. In this review we summarize the
orbital properties of extrasolar planets. One of the most remarkable features
of extrasolar planets is their high eccentricities, far larger than seen in the
solar system. We review theoretical explanations for large eccentricities and
point out the successes and shortcomings of existing theories.
