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The scientists belief. It all started with a tremendous bang. Somewhere in our galaxy a star exploded, throwing out masses of gas and dust. This supernova, as these explosions are called, happened about 5 billion years ago.
The wreckage from the explosion then crashed into a nearby cloud of gas, bringing together the ingredients for our solar system to form.
Little bits of dust began to cluster, making bigger and bigger lumps, and the mixture began to pull together under its own gravity. Eventually the central lump became so hot and dense that it started to generate its own energy, igniting nuclear fires. This was the birth of our sun.
Over millions of years the dust clustered into grains, then lumps, boulders and eventually planetesimals - chunks of rock big enough to have their own gravitational field. Some of these planetesimals became the embryonic forms of the planets in our solar system today.
Slowly these rocky planets began to organise themselves, settling at a comfortable distance from the sun and finding their own orbit. Earth found its path as third planet from the sun. In the early days rocky pile-ups were still common, leaving craters on the surface of all of the planets.
One of these collisions, about 4.5 billion years ago, is thought to have very nearly destroyed the Earth, and was probably responsible for our moon. A large planetesimal, about the size of Mars, gave Earth a glancing blow, chucking a chunk of Earth's crust out into space. Some of the planetesimal merged with Earth, while the ejected lump started its own orbit around Earth and became the moon.
There were no oceans nor continents and no atmosphere. Meteorite collisions, radioactive decay and planetary compression made Earth become hotter and hotter. After a few hundred million years the temperature of Earth reached 2,000C. and Earth's core was formed.
At this point much of the Earth was molten and there may have been a magma ocean at the surface. Gradually the Earth cooled and the planet settled out into a core, mantle and crust. This layering of the planet helped to trigger plate tectonics at the surface, and the Earth began to look a little more like the planet we know today.
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