  Polarimetry is about to become a powerful tool for determining the
atmospheric properties of exoplanets. To provide the basis for the
interpretation of observational results and for predictive studies to guide
future observations, sophisticated analysis tools are required. Our goal is to
develop a radiative transfer tool that contains all the relevant continuum
polarization mechanisms for the comprehensive analysis of the polarized flux
resulting from the scattering in the atmosphere of, on the surface of, and in
the local planetary environment (e.g., planetary rings, exomoons) of
extra-solar planets. Furthermore, our goal is to avoid common simplifications
such as locally plane-parallel planetary atmospheres, the missing cross-talk
between latitudinal and longitudinal regions, or the assumption of either a
point-like star or plane-parallel illumination. As a platform for the newly
developed numerical algorithms, we use the 3D Monte Carlo radiative transfer
code POLARIS. The code is extended and optimized for the radiative transfer in
exoplanetary atmospheres. We investigate the reflected flux and its degree of
polarization for different phase angles for a homogeneous cloud-free atmosphere
and an inhomogeneous cloudy atmosphere. To take advantage of the 3D radiative
transfer and to demonstrate the potential of the code, the impact of an
additional circumplanetary ring on the reflected polarized flux is studied. The
presence of a circumplanetary ring consisting of small water-ice particles has
a noticeable impact on the reflected polarized radiation. In particular, the
reflected flux strongly increases at larger phase angles if the planetary orbit
is seen edge-on because the considered particles tend to scatter forwards. In
contrast, the degree of polarization decreases at these phase angles.
