  Planetary systems consisting of one star and n planets with equal planet
masses \mu and scaled orbital separation are referred as EMS systems. They
represent an ideal model for planetary systems during the post-oligarchic
evolution. Through the calculation of Lyapunov exponents, we study the boundary
between chaotic and regular regions of EMS systems. We find that for n > 2,
there does not exist a transition region in the initial separation space,
whereas for n=2, a clear borderline occurs with relative separation ~ \mu^{2/7}
due to overlap of resonances (Wisdom, 1980). This phenomenon is caused by the
slow diffusion of velocity dispersion (~ t^{1/2}, t is the time) in planetary
systems with n >2, which leads to chaotic motions at the time of roughly two
orders of magnitude before the orbital crossing occurs.
  This result does not conflict with the existence of transition boundary in
the full phase space of N-body systems.
