  The nearest stellar system consists of the stars Proxima, alpha Centauri A
and B and at least one planet Proxima b. The habitability of Proxima b and any
other planets are likely to be significantly influenced by the orbital
evolution of the system. To study the dynamical evolution of the system, we
simulate the motions of Proxima and alpha Centauri A and B due to the
perturbations from the Galactic tide and stellar encounters in a Monte Carlo
fashion. From 100 clones, we find that 74% orbits of Proxima Centauri are bound
to alpha Centauri A and B while 17% and 9% orbits become unbound in the
simulations over the past and future 5 Gyr. If the system migrated outward in
the Milky Way to its current location, more than 50% of clones could become
unstable in backward simulations. The ratio of unstable clones increases with
the simulation time scale and encounter rate. This provides some evidence for a
capture scenario for the formation of the current triple system. Despite large
uncertainties, the metallicity difference between Proxima and alpha Centauri A
and B is also suggestive of their different origin. Nonetheless, further
improvements in the available data and models will be necessary for a reliable
assessment of the history of the Proxima-alpha Centauri system and its impact
on the habitability of Proxima b.
