  Most stars are born in dense stellar environments where the formation and
early evolution of planetary systems may be significantly perturbed by
encounters with neighbouring stars. To investigate on the fate of circumstellar
gas disks and planets around young stars dense stellar environments, we
numerically evolve star-disk-planet systems. We use the $N$-body codes
NBODY6++GPU and SnIPES for the dynamical evolution of the stellar population,
and the SPH-based code GaSPH for the dynamical evolution of protoplanetary
disks. The secular evolution of a planetary system in a cluster differs from
that of a field star. Most stellar encounters are tidal, adiabatic and
nearly-parabolic. The parameters that characterize the impact of an encounter
include the orientation of the protoplanetary disk and planet relative to the
orbit of the encountering star, and the orbital phase and the semi-major axis
of the planet. We investigate this dependence for close encounters ($r_p/a\leq
100$, where $r_p$ is the periastron distance of the encountering star and $a$
is the semi-major axis of the planet). We also investigate distant perturbers
($r_p/a\gg 100$), which have a moderate effect on the dynamical evolution of
the planet and the protoplanetary disk. We find that the evolution of
protoplanetary disks in star clusters differs significantly from that of
isolated systems. When interpreting the outcome of the planet formation
process, it is thus important to consider their birth environments.
