Planetary formation It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that do not condense under gravity to form a protostar. Instead, these remnants become a thin disc of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. Thereafter there still may be many protoplanets orbiting the star or each other.

The designated planetary names are near-universal in the Western world, but some non-European languages, such as Chinese, use their own. A western exception is, naturally, Greece which uses the equivalent Greek gods' names: Hermes (Mercury), Aphrodite (Venus), Gaia (Earth), Ares (Mars), Zeus (Jupiter), Cronus (Saturn), Ouranos (Uranus), Poseidon (Neptune), Pluton—not the expected Hades—(Pluto). Moons are also named after gods and characters from classical mythology, or, in the case of Uranus, after characters from English literature. Asteroids can be named after anybody or anything at the discretion of their discoverers, subject to approval by the IAU's nomenclature panel.