Formation of the solar system. Artwork showing the different sizes of objects (right-hand column) in the solar system at different stages (left-hand column) during its formation. Some 5 billion years ago (top), the primordial solar nebula, a cloud of gas and dust particles began to gravitationally accrete to form larger particles that orbited the newly formed Sun. The particles sizes, from top, are: 0.0001 millimetres; 0.001 millimetres; 0.01 millimetres; 50 kilometres (protoplanets); and 10,000 kilometres (planet-sized objects). The Earth formed about 4.5 billion years ago. The composition of the particles varied, from icy particles (top) to rocky planets with metallic cores (bottom).