  Every Galactic environment is characterized by a stellar density and a
velocity dispersion. With this information from literature, we simulated flyby
encounters for several Galactic regions, numerically calculating stellar
trajectories as well as orbits for particles in disks; our aim was to
understand the effect of typical stellar flybys on planetary (debris) disks in
the Milky Way Galaxy. For the Solar neighborhood, we examined nearby stars with
known distance, proper motions, and radial velocities. We found occurrence of a
disturbing impact to the Solar planetary disk within the next 8 Myr to be
highly unlikely; perturbations to the Oort cloud seem unlikely as well. Current
knowledge of the full phase space of stars in the Solar neighborhood, however,
is rather poor, and thus we cannot rule out the existence of a star that is
more likely to approach than those for which we have complete kinematic
information. We studied the effect of stellar encounters on planetary orbits
within the habitable zones of stars in more crowded stellar environments, such
as stellar clusters. We found that in open clusters habitable zones are not
readily disrupted; this is true if they evaporate in less than 108 years. For
older clusters the results may not be the same. We specifically studied the
case of Messier 67, one of the oldest open clusters known, and show the effect
of this environment on debris disks. We also considered the conditions in
globular clusters, the Galactic nucleus, and the Galactic bulge-bar. We
calculated the probability of whether Oort clouds exist in these Galactic
environments.
