  Modern N-body techniques for planetary dynamics are generally based on
symplectic algorithms specially adapted to the Kepler problem. These methods
have proven very useful in studying planet formation, but typically require the
timestep for all objects to be set to a small fraction of the orbital period of
the innermost body. This computational expense can be prohibitive for even
moderate particle number for many physically interesting scenarios, such as
recent models of the formation of hot exoplanets, in which the semimajor axis
of possible progenitors can vary by orders of magnitude. We present new methods
which retain most of the benefits of the standard symplectic integrators but
allow for radial zones with distinct timesteps. These approaches should make
simulations of planetary accretion with large dynamical range tractable. As
proof of concept we present preliminary science results from an implementation
of the algorithm as applied to an oligarchic migration scenario for forming hot
Neptunes.
