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A theory for the origin of the Earth, as well as of the other planets, should explain the characteristic features of the Solar System like the existance of inner, terrestial planets and outer giant planets. It should also be compatible with the chemical composition of these celestial objects, as well as the composition of smaller bodies, including meteorites, asteroids, and comets.
Nowadays it is scientifically accepted that the origin of the universe can be explained by the theory of the Big Bang. According to this theory, the universe was 'born' around 15 billion years ago from a single point storing all its mass and energy. 3 billion years after the explosion, the universe was a huge dense cloud of hydrogen and helium. Then some irregularities began to appear which gave birth to the firsts galaxies. Giant dark clouds of gas and dust collected near the plane of the galaxies. At some points, where inestabilities took place, stars began to form due to gravitacional collapse.
It is thought that our Solar System was formed around 4600 years ago when a gaseous nebula containing hydrogen, helium and some heavy elements (2%) began a contracting process like gas whirlwinds converging at great speeds. The density and temperature increased to form the Sun, and around it cloud spun a scattered trail of leftover material. The more volatile elements of the near regions of the sun were forced outwards by stellar winds, while the heaviest ones remained to form the inner planets. The volatile elements would form the outer giant planets.
While being the Sun formed, small solid bodies of low density began to form. These tiny meteorites, spinning through space around the Sun, were drawn towards each other by gravity, colliding to remain a single mass. This mass attracted more and more meteorites to it, until thousands of collisions later the rock was large enough to be called planets (Mercury, Venus, Earth and Mars). At the far end of the solar system the huge clouds of volatile elements (mostly hidrogen) wrapped around the rocky cores of Jupiter, Saturn, Uranus and Neptune, forming giant gaseous planets without a solid surface.
For nearly 600 million years the body of the young Earth, hot from the collisions which had created it, continued to be bombarded by meteorites, adding further to its primitive mass and providing yet more heat to keep the Earth's temperature up. Gravity and radioactivity, which kept the interior hot enough, helped to separate the material of the Earth into layers - the heaviest at the centre, the lightest at the surface.
About 4 billion years ago, the rain of meteorites ceased, and the Earth began to cool by convection, with hot material from the deep interior flowing to the surface, then cooling and sinking back again. In the process of cooling, vast quantities of gases escaped from the semimolten surface and formed the Earth's early athmosphere. Some of the gas (water vapour) cooled into liquid and fell back to earth as the first rain. Once the earth's surface had cooled enough not to sizzle raindrops on impact, the vapour became the present day oceans.