The solar system took shape 4.57 billion years ago when it condensed within a large cloud of gas and dust.
Uranus by accident was discovered in the year 1781 and Neptune in the year 1846 after a deliberate search following a theoretical prediction based on observed irregularities in the orbit of Uranus.
Pluto, discovered in 1930 after a quest for a planet anticipated to lie beyond Neptune, was considered a major planet until the year 2006 when it was renamed a dwarf planet by the International Astronomical Union.
Mercury, at 0.4 Astronomical Unit, is the closest planet to the Sun, while Neptune, at 30.1 Astronomical Unit, is the farthest from the sun.
The four inner, terrestrial planets—Mercury, Venus, Earth, and Mars—along with the Moon have average densities in the range of 3.9–5.5 grams per cubic cm, setting them apart from the four outer, giant planets—Jupiter, Saturn, Uranus, and Neptune—whose densities are all close to 1 gram per cubic cm, the density of water.
Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets.​