According to the nebular theory, how did the Kuiper belt form? According to the nebular theory, how did the Kuiper belt form? It is made of planetesimals that formed beyond Neptune's orbit and never accreted to form a planet. It consists of objects that fragmented from the protosun during a catastrophic collision early in the formation of the solar system. It is material left over from the interstellar cloud that never contracted with the rest of the gases to form the solar nebula. It is made of planetesimals formed in the outer solar system that were flung into distant orbits by encounters with the jovian planets. It is made of planetesimals between the orbits of Mars and Jupiter that never formed into a planet.