General Facts about Comets Comets are believed to be remnants from the formation of the solar system, 4.5 billion years ago, and they may contain the primordial mixture of material from which the solar system formed. They also may have supplied some of the water for today's oceans by bombarding the primeval Earth. So learning more about their composition will shed light on the origin and evolution of the solar system and its planets. Whereas the planets have undergone major changes over time from geological, thermal and atmospheric processes, the comets have likely remained much as they were when they first formed. They have been preserved in the icy cold of the solar system's outermost regions.
Comets come from two regions in the outer solar system.
One region is a disk of icy bodies called the Kuiper Belt, located outside the orbit of Neptune. The other region, called the Oort Cloud, is much larger and may extend to 9 trillion miles from the sun, nearly half way to the nearest star, Alpha Centauri. Comets in the Oort Cloud sometimes end up in orbits that take them into the inner solar system when they are jostled out of place by the gravitational field of a star, or a giant molecular cloud, from which new stars form. Scientists believe that is what happened to Hale-Bopp at least 30,000 years ago. The comet currently takes roughly 3,000 years to orbit the sun, so it will take that long to return.