Iron meteorites originate from cores of planetesimals. The oligarchs continue to accrete until planetesimals are exhausted in the disk around them. The asteroid belt formed from the primordial solar nebula as a group of planetesimals. They are believed to be due to clumps (protoplanets and planetesimals) in the circumstellar disk. Although some scientists refer to the asteroids as residual planetesimals, other scientists consider them distinct. The grains eventually may coagulate into kilometer-sized planetesimals. The formation of planetesimals is the biggest unsolved problem in the nebular disk model. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists. During this stage embryos expel remaining planetesimals, and collide with each other. The most primitive planetesimals visited by spacecraft are the contact binary Arrokoth. In later stages terrestrial planet spin is also affected by impacts with planetesimals. Grains eventually stick together to form mountain-size (or larger) bodies called planetesimals. This is how planetesimals are often defined. Planetesimals formed beyond this radius were able to accumulate ice. This enhances the mass of planetesimals fourfold.