1950, the Dutch astronomer/scientist called Jan Oort guessed, that comets come from a huge space, which is around 50,000 times away from the sun, than the earth.
Around a year later, an astronomer called Gerard guessed that there must be some kind of belt around the solar system, which is made of comet-like debris, from the creation of the solar system, and would be situated beyond Neptune. He argued, that is would be very unusual if there wasn't any such area, with objects/debris, because this would prove that there is a very discrete "edge" of the solar system.
Kuiper's theory was proved right 1992, when 1992QB1 was discovered. 1992QB1 is a small world with an area of around 283 km. 1992QB1 orbits the sun once in 291 years with a distance of 43 AU from the sun (Pluto orbits the sun once in 248 years, in a distance of 39,5 AU from the sun)

Shortly lots of small objects were discovered, orbiting the sun. This once again proved Kuipers hypothesis/theory right.
Pluto which is also considered as being inside the kuiper belt, is yet considered as the largest object in kuiper belt, even though some scientist believe that there may be some larger objects further orbiting the sun. However there are some scientist that consider it is very unlikely that this is the case.

Another very interesting world which has been discovered is "2002 LM60" also known as "Quaoar". Quaoar is another frozen world, orbiting the sun. Quaoar's diameter is around 1,300 km, about the size of Charon (Pluto's moon). Quaoar/2002 LM60 orbit the sun, in almost a circle. Astronomers believe that it has the most circular orbit, more than any planet, in the solar system.

The kuiper belt is still very unknown, thus NASA has prepared a kuiper journey. To read about NASA's journey "pluto kuiper express" click here