  We revisit the problem of the existence of KAM tori in extrasolar planetary
systems. Specifically, we consider the $\upsilon$ Andromed{\ae} system, by
modelling it with a three-body problem. This preliminary study allows us to
introduce a natural way to evaluate the robustness of the planetary orbits,
which can be very easily implemented in numerical explorations. We apply our
criterion to the problem of the choice of a suitable orbital configuration
which exhibits strong stability properties and is compatible with the
observational data that are available for the $\upsilon$ Andromed{\ae} system
itself.
