  We derive a criterion for the onset of chaos in systems consisting of two
massive, eccentric, coplanar planets. Given the planets' masses and separation,
the criterion predicts the critical eccentricity above which chaos is
triggered. Chaos occurs where mean motion resonances overlap, as in Wisdom
(1980)'s pioneering work. But whereas Wisdom considered only nearly circular
planets, and hence examined only first order resonances, we extend his results
to arbitrarily eccentric planets (up to crossing orbits) by examining
resonances of all orders. We thereby arrive at a simple expression for the
critical eccentricity. We do this first for a test particle in the presence of
a planet, and then generalize to the case of two massive planets, based on a
new approximation to the Hamiltonian (Hadden, in prep). We then confirm our
results with detailed numerical simulations. Finally, we explore the extent to
which chaotic two-planet systems eventually result in planetary collisions.
