By analyzing the meteors that have struck the Earth, scientists have found that asteroids seem to be composed of particular elements and isotopes in certain abundance, believed to be originated from about 5-6 original parent bodies. The makeup of these meteors suggests that there are two varieties of 'stony' and 'iron/nickel' types. By studying the surface of distant moons and the craters on them that would require a very large planetoid or 'planetessimal' to have impacted on it's surface, we know that there were many such bodies in our young solar system. These planetessimals were large enough to differentiate,' or separate their iron/nickel cores from the crystalline, stony exteriors by the process of slow cooling. This type of differentiation also requires a strong, local gravitational field, also provided by these ancient planetessimals. The physical makeup of the meteors that have broken off of the asteroids in our solar system implies that they were part of these primeval planetessimals rather than small trace matter that failed to form a planet.