Spacing and stability of compact systems

Exoplanet transit surveys have revealed the existence of numerous
multi-planetary systems packed close to their stability limit. This feature
likely emerges from the formation and dynamical history of the system.
Understanding it in detail is thus key to constrain our planet formation
scenarios. While the stability limit has been known empirically for decades,
no theoretical explanation was proposed yet. I present a mechanism driving
the instability of tightly packed system. Based on the chaotic diffusion
along the network of three-planet resonances, it reproduces quantitatively
the timescale of instability obtained numerically over several order of
magnitude in time and planet-to-star mass ratios. I discuss the observational
implications of this model, in particular the expected differences between
Super-Earths and terrestrial planet systems.