The Kuiper Belt is an area beyond the orbit of Neptune where thousands of icy objects are located. In 1992, Jane Luu and David Jewitt of the University of Hawaii discovered the first Kuiper Belt Object (KBO). Thousands of KBOs have been discovered. Their orbits around the sun take from two hundred to several thousand years to complete.

Scientists believe that 4.5 billion years ago after the sun formed, leftover dust and gas formed the planets and other objects in our solar system. Small particles orbiting the new star collide, merge together, and grow in size as they continue to gather more dust and gas. Planetesimals are rocky or icy objects about the size of a few city blocks. Planetesimals can continue to collide and merge. When planetesimals reach about 60 miles in diameter, they become protoplanets. Protoplanets stir up planetesimals near their orbits and continue to add to their mass. Protoplanets can eventually become planets like Earth. Planetesimals grow in a few thousand years and planets can take up to a hundred million years to form. Near the star, rocky planets form. Far away from the star, planets made almost entirely of ice may form.

KBOs are mostly made of ice, forming past the orbit of Neptune. Scientists believe they formed like snowflakes form. Methane and water condense onto small carbon or silicate grains. The grains develop into complex shapes, which collide and form larger objects. They range in size. There are probably about 100,000 small KBOs that range from 60 to 300 miles in diameter, and about 500 KBOs larger than 300 miles across (15 have been found). KBOs are like Pluto and its moon Charon, with small cores of iron and rock surrounded by thick layers of ice. Frozen gases may blanket the KBOs, like Pluto’s gaseous atmosphere frozen as ice on its surface. Pluto and Charon are located in the Kuiper Belt and are probably KBOs too.