The solar system formed approximately 4.6 billion years ago from a gigantic cloud of gas and dust. This stellar nursery nebula would have given birth to hundreds of thousands of other stars some like the Sun living billions of years. Other stars would have been giants that gobbled up their fuel in a few million years. The processes of solar system formation happen in several stages. First, the dust cloud must collapse into protoplanetary discs, each disc giving rise to a new

This image is an artist's impression of a protoplanetary disc, with the prostar in the centre.

solar system. This collapse can either be spontaneous collapse due to the cloud’s own gravity, or it can be triggered by a nearby supernova, for example. Next, tiny dust grains in the disc would begin to stick together through electrostatic attraction, caused by ionisation in the disc. It is still not exactly known how particles the size of 1cm to a few hundred meters stick, as they are too big for electrostatics, but too small for gravity to take effect. However once they have reached a hundred meters or so, gravity takes over and protoplanets begin to form.

Conditions in the early solar system would have been quite different from today, the Sun was much cooler, and the Earth did not yet have a Moon. The formation of the Moon occurred over four billion years ago, when a protoplanet the size of Mars collided with Earth. If life on had already started before this it most certainly would have been wiped out as both protoplanets merged into one. Large amounts of material was ejected into space, eventually coalescing into the Moon.

The cores of giant planets, Jupiter, Saturn, Uranus and Neptune, formed in much the same way as the inner planets. However, in the outer disc, there was a lot more gas and dust per planet than the inner solar system. this is because the circumference of the giant planets' orbits are bigger, and the space between the planets is also greater. This all means that the gravity of the rocky cores attracted vast amounts of gas, which increased their gravity so yet more gas could be attracted until it had all been use up. It is now theorised that Uranus and Neptune may have formed much closer to the Sun than their current location. Computer simulations show that giant planets don't form at these distances from the Sun, because processes occur at much slower speeds as there is less energy due to the cold temperature. These two giants would have migrated to their current location over time, and may have even swapped round.

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