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tg fro a certa itial nfiguration (插bers， wetherill ≈ap; boss 1996; ito ≈ap; tanikawa 1999) a syste is defed as experiencg a close enunter when o bodies approach one another with an area of the larr hill radi otherwise the syste is defed as beg stable henceforward we state that our plaary syste is dynaically stable if no close enunter happens durg the a of our lar syste， about &psn;5 gyr cidentally， this defition ay be replaced by one which an ourrence of any orbital crossg beeen either of a pair of plas takes place this is becae we know fro experience that an orbital crossg is very likely to lead to a close enunter plaary and prolaary systes (yoshaga， kokubo ≈ap; ako 1999) of urse this statent cannot be siply applied to systes with stable orbital renances such as the neptune–pto syste

12previo studies and ais of this research

addition to the vagueness of the ncept of stability， the plas our lar syste show a 插racter typical of dynaical 插os (ssan ≈ap; wisdo 1988， 1992) the cae of this 插otic behaviour is now partly understood as beg a result of renance overlappg (urray ≈ap; holan 1999; lecar， frankl ≈ap; holan 2001) however， it would reire tegratg over an enseble of plaary systes cdg all ne plas for a period verg several 10 gyr to thoroughly understand the long-ter evotion of plaary orbits， sce 插otic dynaical systes are 插racterized by their strong dependence on itial nditions

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