When planets, moons, asteroids, and the like form in a nascent solar system, they are created from whatever raw materials are available. In some places iron and other metals are common, and in other places they are rare. The same is true for all other types of materials: rocks and silica grains, hydrogen and helium gases, and so on. The amount of each of these materials varies with location, primarily with orbital radius (distance from the star). The relative abundances of different materials in an object can thus help us understand where in the solar system that object formed.

It is also possible to look closely at the distribution of a particular element, like oxygen, and from this determine where in the solar system an object was formed. There are different types of oxygen, referred to as isotopes (they vary by the number of neutrons in the atomic nucleus). These isotopes occur in particular ratios (for every ten atoms of type A there are four of type B and one of type C, and so on). These ratios depend on location in which the object formed in the solar system. If two objects have the same isotope ratios, then they must have formed in the same place.