AP Photo

We've known for over half a century that the Earth is 4.5 billion years old, but for people like University of California, Davis geologist Qing-zhu Yin, that number just isn't good enough--they need to determine what digit comes after the '5'. Yin has spent the last fifteen years trying to figure out exactly how our solar system formed--how, over a span of some tens of millions of years, a large chaotic disc of dust and gas turned itself into eight planets in orderly orbit around a central star.