The Effects of Encounters on Planetary Systems in Stellar Clusters
Extrasolar planets are found to be on orbits very different from those of the planets in the solar system. Gas-giant planets have been found on orbits tighter than that of the planet Mercury, often with very high eccentricities. These orbits are surprising; models of planet formation suggest that gas giants should be found on circular orbits with similar size to those of solar-system gas giants.