Excitation and propagation of eccentricity disturbances in planetary systems

The high eccentricities of the known extrasolar planets remain largely unexplained. We explore the possibility that eccentricities are excited in the outer parts of an extended planetary disk by encounters with stars passing at a few hundred astronomical units. After the encounter, eccentricity disturbances propagate inward because of secular interactions in the disks, eventually exciting the innermost planets. We study how the inward propagation of eccentricity in planetary disks depends on the number and masses of the planets and the spacing between them and on the overall surface density distribution in the disk. The main governing factors are the large-scale surface density distribution and the total size of the system. If the smeared-out surface density is approximated by a power law Sigma(r) proportional to r(-q), then eccentricity disturbances propagate inward efficiently for flat density distributions with qless than or similar to1. If this condition is satisfied and the size of the planetary system is 50 AU or larger, the typical eccentricities excited by this mechanism by field star encounters in the solar neighborhood over 5 Gyr are in the range 0.01-0.1. Higher eccentricities (>0.1) may be excited in planetary systems around stars that are formed in relatively dense, long-lived open clusters. Therefore, this mechanism may provide a natural way to excite the eccentricities of extrasolar planets.