The planets can be classified into two major groups: (1) the terrestrial planets of Mercury,Venus,Mars, and Earth and (2) the giant planets of Jupiter, Saturn, Uranus, and Neptune.

Comets are porous aggregates of water ice, frozen methane, frozen ammonia, dry ice, and dust. The solar system is surrounded by the Kuiper Belt and the Oort cloud of these objects. Something nudges one of the icy bodies and it falls into a long elliptical orbit around the Sun. As it approaches the Sun, increased radiation evaporates ices and pushes ions and dust into a long visible tail. Asteroids are rocky or metallic bodies that are mostly located in a belt between Mars and Jupiter. The remnants of comets, fragments of asteroids, and dust are called meteoroids. A meteoroid that falls through Earth's atmosphere and melts to a visible trail of light and smoke is called a meteor. A meteoroid that survives the trip through the atmosphere to strike the surface of Earth is called a meteorite. Most meteors are fragments and pieces of dust from comets. Most meteorites are fragments that resulted from collisions between asteroids.

The protoplanet nebular model is the most widely accepted theory of the origin of the solar system, and this theory can be considered as a series of events, or stages. Stage A is the creation of all the elements heavier than hydrogen in previously existing stars. Stage B is the formation of a nebula from the raw materials created in stage A. The nebula contracts from gravitational attraction, forming the protosun in the center with a fat, bulging accretion disk around it. The Sun will form from the protosun, and the planets will form in the accretion disk. Stage C begins as the protosun becomes established as a star. The icy remains of the original nebula are the birthplace of comets. Asteroids are other remains that did undergo some melting.