The dwarf planets at the edge of the solar system are icy time capsules. They preserve a record of the solar system’s early history — a time when the giant outer planets changed orbits, scattering many of the “leftovers” from their own formation. There are four official dwarf planets — and many more un-official ones — in a region known as the Kuiper Belt — a broad disk of icy bodies billions of miles from the Sun. The best-known member of the belt is Pluto. Most of these bodies probably formed much closer to the Sun, in the realm of the giant planets Jupiter, Saturn, Uranus, and Neptune. Recent studies suggest that when the solar system was still young, these worlds moved around a bit. Jupiter moved toward the orbit of Mars before retreating to its present position. And the most distant planets, Uranus and Neptune, may even have swapped places. During this planetary migration, the gravity of the giant worlds hurled many smaller ones — the leftover building blocks of all the planets — i
