Region of stability of the particles in the space of a versus e for the coplanar case (I_A = i = 0). The plots represent the final results for the orbital configurations I, II and III. The red and green curves give the analytical estimates for the stability region (“protected region”). These curves represent the internal, a_I and outer, a_O, critical semi-major axes. The red and green curves are given by Eqs. (4) and (3), respectively. The dashed curve represent the (a_lim, e_lim) pair for a particle within the “protected region”.