es place. this is because we know from experience that an orbital crossing is very likely to lead to a close encounter in planetary and protoplanetary systems (yoshinaga， kokubo & makino 1999). of course this statement cannot be simply applied to systems with stable orbital resonances such as the neptune–pluto system.
1.2previous studies and aims of this research
in addition to the vagueness of the concept of stability， the planets in our solar system show a character t