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What is the name of the theory that astronomers use to explain why the Asteroid belt and the Kuiper Belt is alot more sparce than when the Solar System just formed?
The Nice Model(Jupiter and Saturn got into a 2:1 orbital resonance, which flung most of the asteroids, scattering them and few asteroids survive in the belt. Even Uranus and Neptune got flung outwards at that time-yes, they were once much closer to the Sun than they are now.)