The Kuiper Belt is a region of space along the ecliptic outside the orbit of Neptune. It contains countless comets and "super comets" or planetesimals. It's existance was predicted in 1951 by Gerald Kuiper, for whom the belt was named.
Starting in March 1992, and in rapid succession thereafter, a series of icy planetesimals, or "super comets" have been discovered orbiting the sun beyond Neptune. These bodies fall into two distinct groups, those having eccentric orbits syncronized at a 3:2 ratio with the orbital duration of Neptune's and those having a larger, more circular orbit. Considering Pluto's small size (smaller than the earth's moon) it should not be considered a planet but the largest known member of the Kuiper Belt. It is interesting to note that Ceres, the largest main belt asteroid is more massive than Pluto. This is due to composition. Ceres is mostly iron and rock while Pluto is made up of mostly ices.
In addition to these Kuiper Belt objects, also known as Transneptunian, asteroids, another group of objects orbit the sun among the giant planets of the outer solar system. These objects, known as Centaur Objects, are most likely former Kuiper Belt objects. The low points of their orbits have fallen deep into the outer solar system much like the near earth asteroids have fallen into the inner solar system from their original orbits in the asteroid belt between Mars and Jupiter. Some of these objects have had their orbits futher modified by the giant planets' gravity fields with their high points now also lowered. Chiron, discovered in 1978, is an example of one of these objects. If one of these objects were to fall into the inner solar system it would be awe inspriring. These bodies are much larger than the major comets seen in historical time.
In addition to plenetesimals discovered in the Kuiper Belt, the Hubble Space Telescope has taken a pair of images of a tiny portion of the belt. By comparing these pictures using statistical analysis, about 30 object the size of comets we are used to seeing have been detected. This verifies the other part of Gerald Kuiper's theory: that the Kuiper Belt was the source of the periodic comets that dazzle us as they pass through the inner solar system.
The Kuiper Belt
Object Distance Diameter Discovered
from sun AU km
Pluto 39.5 2,330 1930
Charon 39.5 1,200 1978
1992 QB1 43.9 283 1992 Aug
1993 FW 43.9 286 1993 Mar
1993 RO 39.3 139 1993 Sep
1993 RP 39.3 96 1993 Sep
1993 SB 39.4 188 1993 Sep
1993 SC 39.5 319 1993 Sep
1994 ES2 45.3 159 1994 Mar
1994 EV3 43.1 267 1994 Mar
1994 GV9 42.2 264 1994 Apr
1994 JQ1 43.3 382 1994 May
1994 JR1 39.4 238 1994 May
1994 JS 39.4 263 1994 May
1994 JV 39.5 254 1994 May
1994 TB 31.7 258 1994 Oct
1994 TG 42.3 232 1994 Oct
1994 TG2 41.5 141 1994 Oct
1994 TH 40.9 217 1994 Oct
1994 VK8 43.5 273 1994 Nov
1995 DA2 36.3 1995 Feb
1995 DB2 46.3 1995 Feb
1995 DC2 44.0 1995 Feb
1995 FB21 42.4 1995 Mar
1995 GJ 42.9 1995 Apr
1995 GA7 39.5 1995 Apr
1995 GY7 41.3 1995 Apr
1995 HM5 39.4 1995 Apr
1995 KJ1 43.5 1995 Jun
1995 KK1 39.5 1995 Jun
1995 QY9 39.5 1995 Sep
1995 QZ9 39.5 1995 Sep
1995 WY2 47.6 1995 Dec
1995 YY3 39.4 1995 Dec
The Centaur Objects
Object Distance Orbital
from sun Duration
AU year
Hildalgo 5.8 14.0
Chiron 13.7 51.0
Pholus 20.4 92.4
1991 DA 11.8 41.0
Plot of the Outer Solar System