Match the following vocabulary to the descriptions.

1. protoplanet one of the small celestial bodies that, according to one theory (planetesimal hypothesis) were fused together to form the planets of the solar system

2. primordial pertaining to or existing at or from the very beginning

3. planetesimals composed of parts or elements that are all of the same kind

4. accretion something produced in the making of something else

5. homogeneous the processes that cause an essentially homogeneous accreted body made up of primordial solar material to become separated into layers having different chemical and/or physical properties

6. planetary differentiation the collection of matter, in the process of condensation, from which a planet is formed.

7. by-product the formation of planets and other bodies by collection of material through gravity.