A little over 4 billion years ago, the planets in our solar system coexisted with vast numbers of small rocky or icy objects orbiting the Sun. These were the last remnants of the planetesimals – the primitive building blocks that formed the planets. Most of these leftover objects were then lost, as shifts in the orbits of the giant planets scattered them to the distant outer reaches of the solar system or beyond. But some were captured in two less-distant regions, near points where the gravitational influence of Jupiter and the Sun balance, and have remained trapped there, mostly untouched, for billions of years.