 Most models of Solar System formation show both KBOs and SDOs first forming in a primordial belt, with later gravitational interactions, particularly with Neptune, sending the objects outward, some into stable orbits (the KBOs) and some into unstable orbits, the scattered disc.Due to its unstable nature, the scattered disc is suspected to be the point of origin of many of the Solar System's short-period comets.Their dynamic orbits occasionally force them into the inner Solar System, first becoming centaurs, and then short-period comets.