How The Earth Was Created

The Earth was created 4.54 billion years ago when part of the Sun's accretion disc agglomerated into a spherical body. At that time, shortly after the birth of the Sun itself, the solar system's matter was much more scattered around, in the form of asteroids and dust rather than planets. This "matter cloud" has been called a Bok globule, and these globules have been observed in other parts of the Galaxy. Scientists have determined the age of the Earth relatively precisely using isotope dating of the world's oldest rocks.

It is not known precisely what mechanism caused the precursor of the solar system, a gaseous nebulae, to form into the Sun and its attendant accretion disc. It may have been shockwaves from a nearby supernova, or simple gravitational collapse due to a threshold density. Whatever the cause, when enough density gathered in the center of the gas cloud, it ignited to become the Sun. The resulting heat banished volatiles (materials with low melting points) to the outer solar system, while leaving rocky bodies, like the Earth, Mercury, Venus, and Mars, in the inner solar system, where they could grow.

http://www.wisegeek.com/how-was-the-earth-created.htm