Question: why did the earth come into existance

Simon Langley-Evans answered on 12 Nov 2013:

Wow, these astronomy questions are testing my biologists brain to the full. Luckily astronomy is one of my hobbies.

Our sun was born out of a big cloud of gas about 4.5 billion years ago. When it formed there was a huge cloud of dust and small rock surrounding it and whirling around it due to the gravity of the sun. These bits of rock and dust crashed into each other and as they did so they began to stick together and form larger and large clumps. Some of these clumps were big enough to have gravity of their own and pulled in even more rock and dust to make them larger still. Eventually these clumps were big enough to be the size of asteroids and planets. Some of those crashed into each other and either broke into small bits that got hoovered up by the sun or the bigger planets, but some stuck together to form giant planets like Jupiter.

The early Earth got hit by a big object that could have formed a planet itself. The collision was massive and the big lump that blasted off the Earth went on to be our Moon.

If you think about how the solar system is today, we have the four rocky planets nearest the sun (Mercury, Venus, Earth and Mars) and then before the gas giants (Jupiter, Saturn, Uranus, Neptune) there is the asteroid belt. The asteroids are all that is left of the original disk of dust and rock that the planets were formed from. None of the asteroids got big enough to become planets.