Kuiper Belt Objects

In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth is.

A year later astronomer Gerard Kuiper suggested that some comet-like debris from the formation of the solar system should also be just beyond Neptune. In fact, he argued, it would be unusual not to find such a continuum of particles since this would imply the primordial solar system has a discrete "edge."

Several Kuiper Belt objects have been discovered since including 1992 QB1 and 1993 SC. They appear to be small icy bodies similar to Pluto and Triton (but mostly smaller). There are more than 800 known trans-Neptunian objects.

Many orbit in 3:2 resonance with Neptune (as does Pluto). Color measurements of some of the brightest have shown that they are unusually red. In late 2002, a Kuiper Belt object over 1000 km in diameter was discovered and provisionally designated 2002 LM60 "Quaoar".

In early 2004 an even larger one, 2004 DW, was found (its size isn't well known yet, but it's almost certainly smaller than Pluto). And in late 2005 the discovery of 2003 UB313 (now officially named "Eris") was announced; it is very likely somewhat larger than Pluto.

Due to these discoveries, on August 24, 2006, the IAU announced a first-ever definition of 'planet', and these large Kuiper belt objects accordingly became known officially as dwarf planets. A number of astronomers around the world came out in public disagreement with the definition in the days following it.

It is estimated that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number (and mass) of similar sized objects in the main asteroid belt.
Neptune's moon Triton is commonly thought to be a captured KBO.