In 1950 Jan Oort noticed that most comets had their aphelia, furthest distance from the sun, at about 50,000 times the distance from the Sun to the Earth, or astronomical units. Since the comets approached from any old direction, he proposed that they resided huge swarm in a shell 50,000 AU from the Sun. The gravity binding the comets to the Sun is so weak at this range than about any disturbance, such as a passing star, shakes a few of them loose to fall inward toward the Sun. This is how we currently explain the long period comets, those that show up once and then never again for hundreds of years.<br /><br />Short period comets behave differently though. They show up periodically, every 200 years or less, and they approach on the same plane or disk that the planets occupy. The do not approach from above or below the disk as the long period comet do. They have their aphelia at perhaps only 50 AU, about as far out as the furthest planet Pluto is. It appears that they are leftover material from the formation of the solar system.<br /><br />Twelve years ago we started finding the fist residents of this Kuiper Belt with the discovery of 1992 QB1. We've found a bunch more, which is amazing when you consider that you're looking for something the color of coal at maybe 4 or 5 billion miles away. More recently we have seen debris fields around other stars that are similar to our own Kuiper Belt.<br /><br />Our current best explanation for the more distant Oort Cloud is that all these objects were originally part of the Kuiper Belt and intermixing with the outer planets. As they came near the larger ones, especially Jupiter, the gravity tossed them into new orbits. When those orbits took them inside the solar system, then they eventually ran into a planet and disappeared from the scene. When they went outside the solar system then the took up residence where the Oort Cloud is now.