Reply #2: Sounds pretty unlikely to me [View All]

given the distances we're talking about and how a close enough encounter with another system would distort orbits enough to destroy both systems.

I find it likelier that there was a planet between Mars and Jupiter that suffered a catastrophic collision of some sort during its formation.

To capture anything into a stable orbit it must be moving slowly enough and it can't come close to any of the existing planets. Quite posibly the outer planets like Pluto and the posited X Y and Z planetoids are from outside the system, and quite possibly whatever destroyed the fifth planet was, but it's highly unlikely that all the material in the asteroid belt is from outside the system. The orbit is just too in line with the other system planets.