BUILDING

After the Big Bang, it took the Universe a billion years to evolve into an elaborate tapestry of galaxies and stars. Over the next 12 or so billion years, more stars and galaxies gradually weaved themselves into existence, reincarnated from the dying embers of elderly predecessors.

Our Solar System is a relative newcomer in this lengthy tale of cosmic creation. A drifting gas cloud on the very edge of the Milky Way slowly began to shrink and spin round. This cloud, or nebula, gradually contracted to a disc about the size of Neptune's orbit, and as it did so it heated up. Once the nebula had warmed to a few thousand degrees, it began to separate into two different clouds. The scorching centre continued heating until eventually the Sun exploded into action about 5 billion years ago. The rest levelled out to form a revolving disc that gradually cooled down.

As the temperature dropped, the gas quickly condensed into tiny solid particles of rock, metal and ice. As they smashed into each other inside the spinning disc they stuck together to form pebbles. These pebbles rapidly fused to form rocks, then boulders and eventually, after around 100 million years, a set of nine complete planets in stable orbits.

The characteristics of each of the planets also reflect their position in this cosmic cooking pot. Around the inner region near the scorching Sun the planets are made from toughened rock and metal, because all the lumps of ice were vaporised and blown away. Later, as the solar system cooled, these ejected gases condensed and clung back onto the surface of a few of the farther planets, such as our Earth.

The cooler outer planets managed to keep hold of more of this vapour, which clung to their rocky cores, forming the gas giants like Jupiter and Saturn. Right out at the barren edge of the Solar System, as it gradually merges into the freezing void of space, the icy remnants congregated. It is here that the comets reside, the frozen balls of ice and rock that light up as they swing past the Sun on their grand orbits.