Almost 5 billion years ago, an insignificant cloud of gas on the fringe of an ordinary spiral galaxy began to collapse. Its centre started to glow as a star formed; the disc of gas and dust around it coagulated into smaller bodies in orbit. Such a scene had been played out billions of times before in this and countless other galaxies. But this was a special occasion in one respect: intelligent beings would one day emerge on one of these smaller planets. They would call their home Earth, and would wonder how their star – the Sun – and its family of planets had come into being.
The creation myths of the world’s religions naturally linked the creation of the Earth with the birth of the Universe as a whole. Since the 17th century, astronomers have tried to take a more rational stance, but they have faced one serious problem: there is only one Solar System. While other scientists can compare and contrast different specimens to find what is normal and what is a deviation, we do not even know if planetary systems like ours are common in the Galaxy. If many stars have planets, then the birth of planets is a natural by-product of the formation of a star and a theory of planet formation should follow on from theories of star birth. At the other extreme, our Solar System might be unique, created by some freak event.
At the end of the 19th century, astronomers thought that the planets owed their existence to a chance brush between the Sun and another star. The close encounter drew a streamer of gas…
