We don't know exactly what kick-started this process. Perhaps, with pleasing symmetry, it was the shock wave from the explosive death throes of a nearby star. It was not, at any rate, a particularly unusual event. It had happened countless times since the Milky Way itself came into existence about 13 billion years ago, and in our telescopes we can see it still going on in distant parts of our galaxy today. As stars go, the sun is nothing out of the ordinary.
And yet, as far as we know, it is unique. From a thin disc of stuff left over from its birth, eight planets formed, trapped in orbit by its gravity. One of those planets settled into a peculiarly tranquil relationship with its star and its fellow planets. Eventually, creatures emerged on it that began to wonder how their neighbourhood came to be as it is - and could formulate the following six enduring mysteries of our familiar, and yet deeply mysterious, solar system.