Our earth is roughly 4.55 billion years old. This figure is derived from scientific analysis of meteorites and minerals on earth.

How did the Earth form?

Our understanding of the formation of the Earth is not complete and there is still much work to be done in this area. However, the most successful theory so far goes as follows:

Our solar system began to form from a cloud of gas and dust. Such clouds, formed from the explosions of stars, are relatively common in the universe and are called nebulae. Particles of dust were pulled together by gravity to form increasingly large and dense lumps. At the centre of the nebula, enough material accumulated to form a star - our sun. Because of its gravity, the force that pulls all objects together and its spin, it reshaped the nebula into a wide rotating disc.

Inside the disc, lighter materials were driven further away from the sun than heavier ones by solar wind - a stream of charged particles that flows out of the sun in all directions. This is why the inner planets, Mercury, Venus, Earth and Mars, are all made of heavier stuff (mostly rock and metal) than the outer planets. They were formed as particles of dust, gas, rock and ice continued to accumulate in the disc because of gravity.

At first the Earth was a very hot, glowing ball of molten rock held together by gravity, but over millions of years the surface cooled to form a thin rocky crust. Gases trapped by gravity formed an early atmosphere which consisted of poisonous chemicals like methane.

Where does the water come from?

Most of the Earth's water probably came originally from comets - clumps of rock, dust and ice, like dirty snowballs hurtling through space. Sometimes they hit the Earth and the water in them converts to liquid or steam. While the Earth was a fiery ball it may have been surrounded by water vapour until the surface cooled enough to allow the water to condense.

Where does the moon come from?

The moon is generally though to have been formed by the impact of a large asteroid with the early Earth. A huge cloud of debris was thrown into space which condensed under gravity to form the moon.

See also: the Precambrian