A montage of the planets and the Earth's Moon. These images were
taken by a variety of spacecraft. Source: Courtesy NASA/JPL-
Caltech The Sun and the planets formed around 5 billion years
ago, from a cloud of gas and dust left behind by dying stars.
This was mostly Hydrogen and Helium, but there were traces of
the heavier elements which would allow the formation of solid
planets and, eventually, life. The cloud collapsed into a
rotating disk, with almost all of the material concentrated in a
central bulge, where the Sun would form. At certain distances
out from the centre, mass started to accumulate and gravity
pulled in more and more material. The planets were beginning to
form. They swept the area around them clear of small pieces of
dust and rock as they orbited. They also frequently collided
with larger rocks, growing in the process. Eventually they
reached their current size. The material left became the comets
and asteroids. For another billion years or so stray comets and
asteroids continued to bombard