  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 hundreds of AU. After the encounter, eccentricity disturbances propagate
inward due to 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 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)\propto r^{-q}, then eccentricity disturbances propagate
inward efficiently for flat density distributions with q < 1. 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.
