As the planets of the Solar System took shape by accretion from dust and gas, the leftover material was scattered throughout the same disc. The young giant planets — Jupiter and Saturn especially — dictated what happened to it. Their gravity was strong enough either to fling icy bodies inward toward the Sun or to push them out to the system's furthest reaches.

That outward sweeping left two main structures. Just beyond Neptune's orbit, between roughly 30 and 50 astronomical units, lies a relatively flat torus: the Kuiper Belt. Pluto lives here, along with billions of smaller icy bodies. Much farther out — between about 2,000 and 100,000 AU — sits a spherical cloud entirely surrounding the Sun: the Oort Cloud. The objects within it wait at the very edge of the Sun's gravitational reach, on orbits that take hundreds of thousands of years to complete.

These two reservoirs are the archaeological record of the Solar System. The short-period comets we see today come from the Kuiper Belt; the long-period ones from the Oort Cloud. Every comet that swings inward carries with it a piece of the early Solar System — material flung from the disc 4.5 billion years ago and preserved untouched ever since.