There is also no exact dividing line between a planetesimal and protoplanet. After the planetesimal solidified, it broke up in a collision with another planetesimal. A proplyd may form new planets and planetesimal systems. Interactions between Pluto-massed objects in the outer planetesimal disk can result in an early instability. Taken together, these facts suggest that Lutetia should be classified as a primordial planetesimal. Several percent of the initial planetesimal disc can be deposited in these reservoirs. Gravitational encounters between the planets then scatter Uranus and Neptune outward into the planetesimal disk. Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again. Some evidence exists that planetesimal formation may have bypassed these barriers to incremental growth. This caused a feud with Forest Moulton, who co-developed the planetesimal hypothesis. The exoplanet systems discovered have either no planetesimal systems or one very large one. The Solar System has unique planetesimal systems, which led the planets to have near-circular orbits.