  The orbits in the HD 160691 planetary system at first appeared highly
unstable, but using the MEGNO and FLI techniques of global dynamics analysis in
the orbital parameter space we have found a stabilizing mechanism that could be
the key to its existence. In order to be dynamically stable, the HD 160691
planetary system has to satisfy the following conditions: (1) a 2:1 mean motion
resonance, combined with (2) an apsidal secular resonance in (3) a
configuration $P_{c}(ap) - S - P_{b}(ap)$ where the two apsidal lines are
anti-aligned, and (4) specific conditions on the respective sizes of the
eccentricities (high eccentricity for the outer orbit is in particular the most
probable necessary condition). More generally, in this original orbital
topology, where the resonance variables $\theta_{1}$ and $\theta_{3}$ librate
about $180^{\circ}$ while $\theta_{2}$ librates about $0^{\circ}$, the HD
160691 system and its mechanism have revealed aspects of the 2:1 orbital
resonances that have not been observed nor analyzed before. The present
topology combined with the 2:1 resonance is indeed more wide-ranging than the
particular case of the HD 160691 planetary system. It is a new theoretical
possibility suitable for a stable regime despite relatively small semi-major
axes with respect to the important masses in interactions.
