  The GJ876 system was among the earliest multi-planetary detections outside of
the Solar System, and has long been known to harbor a resonant pair of giant
planets. Subsequent characterization of the system revealed the presence of an
additional Neptune mass object on an external orbit, locked in a three body
Laplace mean motion resonance with the previously known planets. While this
system is currently the only known extrasolar example of a Laplace resonance,
it differs from the Galilean satellites in that the orbital motion of the
planets is known to be chaotic. In this work, we present a simple perturbative
model that illuminates the origins of stochasticity inherent to this system and
derive analytic estimates of the Lyapunov time as well as the chaotic diffusion
coefficient. We then address the formation of the multi-resonant structure
within a protoplanetary disk and show that modest turbulent forcing in addition
to dissipative effects is required to reproduce the observed chaotic
configuration. Accordingly, this work places important constraints on the
typical formation environments of planetary systems and informs the attributes
of representative orbital architectures that arise from extended disk-driven
evolution.
