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The main asteroid belt, which inhabits a relatively narrow annulus -2.1-3.3 Au from the Sun, contains a surprising diversity of objects ranging from primitive ice-rock mixtures to igneous rocks. The standard model used to explain this assumes that most asteroids formed in situ from a primordial disk that experienced radical chemical changes within this zone (1). Here we show that the violent dynamical evolution of the giant-planet orbits required by the so-called Nice mode (2-4) leads to the insertion of primitive trans-Neptunian objects into the outer belt. This result implies that the observed diversity of the asteroid belt is not a direct reflection of the intrinsic compositional variation of the proto-planetary disk. The dark captured bodies, composed of organic-rich materials, would have been more susceptible to collisional evolution than typical main-belt asteroids. Their weak nature makes them a prodigious source of micrometeorites--sufficient to explain why most are primitive in composition and are isotopically different from most macroscopic meteorites (5,6)., Our result that a significant fraction of objects in the main asteroid belt are captured primordial trans-Neptunian objects (hereafter termed 'comets' or 'comet-like' to reflect their primitive, icy natures) comes [...]

Standard planet-formation models have been unable to reconstruct the distributions of the Solar System's small, rocky planets and asteroids in the same simulation. A new analysis suggests that it cannot [...]