Named for Gerard Kuiper, one of the first people to posit its existence, the Kuiper Belt consists of uncounted ice-rock bodies confined to the plane of the Solar System and extending outwards from the orbit of Neptune. The short-period Jupiter-type and Halley-type comets originate within it, while long-period comets have their home much further out in the Oort cloud.

The existence of the Kuiper Belt was confirmed in 1992 with the discovery of the first Kuiper Belt Object (KBO). Since then, almost 1,000 more KBOs have been discovered, with the largest having diameters equivalent to or greater than that of Pluto. In fact, it is now believed that Pluto is not a planet at all, but rather merely one of the large KBOs that populate the outer regions of the Solar System.

Current theories have the Kuiper Belt forming along with the rest of the Solar System, though it is still a matter of debate whether it formed in place or was pushed out to its present position as Neptune migrated outwards. If it did form in situ, then the region has clearly been heavily depleted, since it contains less than 1% of the mass needed to build the large objects observed.

The Kuiper Belt begins at the orbit of Neptune and extends out from there. All classical Kuiper Belt objects have semi-major axes of less than 50 AU, suggesting a distinct outer edge to the belt at this distance.
Credit: NASA and A. Feild (STScI)

Thus far, all the KBOs discovered have perihelia of less than 50 AU, though scattered disk KBOs have eccentric orbits which take them much further from the Sun. This suggests that there is a distinct outer edge to the Kuiper Belt at 50 AU, an unexpected observation based on theories of how the Kuiper Belt formed. There are three main theories to account for this abrupt truncation of the Kuiper Belt:

1. it is an observational effect. KBOs exist beyond this distance, but are too small to be detected with current instrumentation
2. the belt was tidally truncated during its formation by a passing star
3. the edge of the belt marks the maximum distance that objects were transported by Neptune's migration

Deeper surveys for faint KBOs should have been able to detect objects beyond 50 AU and have cast the first theory into doubt. The second theory requires the star to have passed within 150 AU of the Sun, and while the average distance between stars in the current environment of the Sun is more like 200,000 AU, if the Sun formed in a dense cluster, this could have occurred. This leaves the third theory as the most probable explanation for the Kuiper Belt truncation at this time.