Planetary migration not only make the formation of Uranus and Neptune possible, but it also explains why the Kuiper belt is not nearly as massive as it should. Also the various resonances in Kuiper belt and the main asteroid belt can be explained by moving planets, as well as Trojan asteroids. Since closely-orbiting extrasolar planets must have migrated, this doesn't sound at all far-fetched. When Saturn's orbit became unstable, our Solar System must have looked very strange with many eccentric orbits.

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman