The Solar System — Know

Why are the four planets nearest the Sun small and rocky while the four furthest are large and gaseous, according to the accretion model of Solar System formation?

The four planets closest to the Sun (Mercury, Venus, Earth, Mars) are small and rocky, while the four furthest planets (Jupiter, Saturn, Uranus, Neptune) are large and gaseous. This is explained by the accretion model of Solar System formation: (a) the model depends on gravity - the Solar System formed from a collapsing interstellar cloud, with gravitational attraction pulling particles of gas and dust together (accretion) to build up the Sun and planets. (b) The original interstellar cloud contained many different elements, including heavier elements that form rock and metal as well as light gases such as hydrogen and helium and ices. (c) As the cloud collapsed under gravity it began to rotate faster and flattened into a spinning accretion disc around the young Sun. Close to the Sun it was too hot for gases and ices to condense, so only rock and metal could accrete, forming small, dense, rocky planets; further out it was cool enough for gases and ices to condense too, so much more material was available to accrete, forming large gaseous (giant) planets.