  We consider planetary systems evolving under the effect of a Stokes-type
dissipative force mimicking the outcome of a type II migration process. As
inward migration proceeds and the planets follow the circular family (they
start on circular orbits) and even though they are initially almost coplanar,
resonance capture can be realized. Then, at the \textit{vertical critical
orbits} (VCOs), that the circular family possesses, the inclination excitation
can abruptly take place. The planets are now guided by the spatial elliptic
families, which bifurcate from those critical orbits. We herein, perform a
direct link of mutually inclined stable planetary systems on circular orbits
trapped in \textit{mean-motion resonance} (MMR) with the existence of VCOs of
high values of multiplicity. It is shown that the more the multiplicity of the
periodic orbits of the circular family increases, the more VCOs (corresponding
to more MMRs) appear. In this way, we can provide a justification for the
existence of resonant planets on circular orbits, which could, even further to
that, evolve stably if they were mutually inclined.
