Kuiper Belt
The Kuiper Belt is a ring of icy objects that exist in sporadic orbits beyond the orbit of Neptune. They are not gathered together tightly but spread out across millions of miles. Their orbits are sporadic and occasionally disrupted by Jupiter and Saturn. This disruption can cause the Kuiper Belt objects to go out of their orbit and fly out of the solar system. It can also cause the objects to go inward toward the sun. There the objects can get caught in the sun's gravitational pull and become comets.
The KBO
In 2009, the Hubble Space telescope detected a Kuiper Belt object that has been labeled the KBO. This object is the smallest natural object detected in the solar system. It is the smallest object that has ever been found by a telescope and seen with the eye. The Hubble is capable of detecting smaller objects, but as of June 2011 it hasn't found an object smaller. It is about 4.2 billion miles away from the Earth. If you were to travel by car at a rate of 60 miles per hour, it would take you 70 million hours, 2,916,666 days or 7,990 years to reach the KBO.
Size
The KBO is only 3,200 feet across, making it less than a mile wide. It is just a little over half a mile wide. The smallest object previously seen by the Hubble was another Kuiper Belt object. This Kuiper Belt object was 30 miles across, which made it 50 times larger than the current smallest object. Compare this to the size of Earth, which is 8,000 miles across, making the KBO 8,000 times smaller than the Earth.
Other Belts and Formation
The Hubble has noticed other Kuiper Belt objects circling other stars. They have only been able to make out much larger objects, finding nothing as small or smaller than the KBO. Scientists theorize that these objects are leftover debris from when the planets were formed. The debris in these belts simply didn't get integrated into the creation of planets in the way that the rest of the material in the solar system did. Scientists also theorize that these objects could collide and grind against one another if they got near enough. These collisions could very likely cause objects, such as the KBO, to form.