  On timescales that greatly exceed an orbital period, typical planetary orbits
evolve in a stochastic yet stable fashion. On even longer timescales, however,
planetary orbits can spontaneously transition from bounded to unbound chaotic
states. Large-scale instabilities associated with such behavior appear to play
a dominant role in shaping the architectures of planetary systems, including
our own. Here we show how such transitions are possible, focusing on the
specific case of the long-term evolution of Mercury. We develop a simple
analytical model for Mercury's dynamics and elucidate the origins of its short
term stochastic behavior as well as of its sudden progression to unbounded
chaos. Our model allows us to estimate the timescale on which this transition
is likely to be triggered, i.e. the dynamical lifetime of the Solar System as
we know it. The formulated theory is consistent with the results of numerical
simulations and is broadly applicable to extrasolar planetary systems dominated
by secular interactions. These results constitute a significant advancement in
our understanding of the processes responsible for sculpting of the dynamical
structures of generic planetary systems.
