This is a drawing which shows where the Kuiper Belt may be located.
Click on image for full size
Beyond the orbit of Pluto is a region which contains more planetismal-sized objects. The objects are usually very small, 10-50 km across, and not very bright. It takes hundreds of years for these objects to complete an orbit around the sun. Because they move so slowly, detecting them is difficult. Where these objects came from and how they got there is still a mystery, but they probaby condensed with the planets as part of the original solar nebula. They may even be the remnants of planetismals themselves! The region is called the Kuiper Belt, after Gerard Kuiper who postulated its existence in 1951.
This picture shows the region of space where the Kuiper Belt is postulated to exist. In this picture, the observer is looking down on the solar system from above. The picture shows a region from the sun to 120 AU. The earth is so close to the center of the drawing that it does not show up.
The first object in the Kuiper Belt was discovered in 1992, proving that Gerard Kuiper's theory of 1951 was correct. Since then many objects have been discovered in this region, which is sometimes called "trans-Neptunian space" because even the planet Pluto could really be a Kuiper Belt object.
This is page 16 of 60