Are retrograde resonances possible in multi-planet systems?

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  • 8 January 2008
Abstract
Most of multi-planetary systems detected until now are characterized by hot-Jupiters close to their central star and moving on eccentric orbits. Hence, from a dynamical point of view, compact multi-planetary systems form a particular class of the general N-body problem (with N >3). Moreover, extrasolar planets are up to now found in prograde orbital motions about their host star and often in mean motion resonances (MMR). In the present paper, we investigate theoretically in a first step a new stabilizing mechanism particularly suitable for compact two-planet systems. Such a mechanism involves counter-revolving orbits forming a retrograde MMR. In a second step, we study the feasibility of planetary systems to host counter-revolving planets. In order to characterize dynamical behaviors of multi-dimensional planetary systems in the vicinity of observations, we apply our technique of global dynamics analysis based on the MEGNO indicator (Mean Exponential Growth factor of Nearby Orbits) that provides the fine structure of the phase space. We also fit a few examples involving counter-revolving configurations by using the Pikaia genetic algorithm. Studying and fitting a particular case, namely the HD73526 planetary system, we find that counter-revolving configurations may be consistent with the observational data. We also point up the novel fine and characteristic structure of retrograde MMRs. We show that retrograde resonances and their resources open a family of stabilizing mechanisms involving new behaviors of apsidal precessions. Considering two possible mechanisms of formation (free-floating planets and the Slingshot model), we may conclude that counter-revolving configurations may be considered as feasible.

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