Nine billion years after the Big Bang the building blocks of the Solar System were present as a huge molecular cloud, several light years across. As parts of the cloud began to collapse there is strong evidence that there were several supernovae near to where the Sun eventually formed. The remnants of these short lived stars and their resulting explosions would create regions of over-density within the cloud, potentially leading to the collapse of material due to gravity and so the eventual birth of the Sun as a protoplanetary disc.



As the disc spins due to momentum, molecular collisions in the centre start to become more common, and the heat starts to rise. As more and more matter began to collect in the middle the Sun started to take shape as an extremely hot and dense protostar. Over time the mass and heat reached the point where nuclear fusion was possible, and the Sun as we know it today was formed, complete with a relatively small amount of leftover material that would go on to form the planets.

The formation of planets is again down to gravity. As the dust particles began to collect to form large rocks they would grow slowly larger through collisions. These early formations are known as planetesimals. The outer solar system was the only area where it is cold enough for molecules such as methane and water to condense, and so the inner planets are formed from heavier rocky elements. These elements are relatively rare and so the inner planets could not grow as large as the gas giants that would eventually form from lighter elements further out in the Solar System.

Initially there were up to a hundred small planets orbiting the Sun. Over the next hundred million years these would collide and merge to form the four rocky planets and their moons that we have today. It is thought that the Earth's moon was formed from a collision with a Mars sized object towards the end of this period. Around four billions years ago the Earth and other terrestrial planets would undergo what is known as the Late Heavy Bombardment. This is a period in time that lasted for several hundred million years where the inner planets where peppered with meteorites and asteroids. The high number of impacts is credited to the migration of the gas giants causing gravitational instability in the Solar System. Evidence of this is present in the highly cratered surface of the Moon and Mercury as well as several large craters on Earth.

Once the Late Heavy Bombardment was over it is believed that solar winds will have pushed much of the excess material out into interstellar space and that the Solar System settled into what we see today, with nine planets and an asteroid belt separating the gas giants from the rocky inner planets.

During and shortly after this time the Earth was very different to what it is now. Known as the Hadean eon the world was hot, dry and extremely volcanic. There were no oceans or oxygen in the atmosphere and the surface was molten. Due to this liquidity the heavier elements fell to the centre of the planet while lighter ones would remain nearer the surface. This process would eventually give rise to the layered structure we see in Earth today.

The next events on Earth would eventually give rise to the planet being hospitable for life. As gravity allowed more of an atmosphere to be retained temperatures would fall to such a level that the outer layer of the planet cooled and a rocky crust was formed. As gases were released from volcanoes and asteroid impact the Earth built up a significant atmosphere that contained water. As clouds formed and rain began to fall the oceans filled up. Within 750 million years the Earth had oceans of water and a rocky crust, although there was significantly less land then than there is now. While the atmosphere didn't yet contain an ozone layer and very little oxygen the stage was set for the evolution of life. There was liquid water, a stable temperature and enough organic chemicals in circulation that would allow bacteria to thrive.