The Solar System formed from a disc of and that gathered due to gravity. The largest mass formed the Sun which became a , and finally lit. As the Solar system formed, it started to turn and formed a flattened . Dust particles came together under the pull of to form rocks. It is believed that the solar wind from the newly lit sun blew the gases outwards.

The four inner planets (Mercury, , Earth, and Mars) are . They consist of a rocky outer layer around an iron and nickel core.

Between the rocky planets and the gas giants there is the belt.

Then there are four gas giants (Jupiter, , Uranus, and ), where it is cold enough for gases to condense. These planets are basically balls of gas. Deeper down the pressure is such that the gases are . In Jupiter, it is believed that hydrogen exists as a . There is a small rocky core. The density of Saturn is so low that if there were an ocean big enough, Saturn would .

These eight planets all orbit the Sun in an direction (when viewed from above their North poles).

is a tiny double planet. Some scientists have decided it's too small to be a planet; it has been reclassified as a .

A planet, Sedna, was discovered in 2003.