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 \geq 3 , there does not exist a transition region in the initial separation space , whereas for n = 2 , a clear borderline occurs with relative separation \sim \mu ^ { 2 / 7 } due to overlap of resonances ( Wisdom , 1980 ) .
This phenomenon is caused by the slow diffusion of velocity dispersion ( \sim t ^ { 1 / 2 } , t is the time ) in planetary systems with n \geq 3 , 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 .