Since the first discovery in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70 000 KBOs over 100 km in diameter are believed to reside there. The Kuiper belt was initially believed to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the Kuiper belt is dynamically stable, and that it is the farther scattered disc, a dynamically active region created by the outward motion of Neptune 4.5 billion years ago, that is their true place of origin. Scattered disc objects such as Eris are KBO-like bodies with extremely large orbits that take them as far as 100 AU from the Sun. The centaurs, comet-like bodies that orbit among the gas giants, are believed to originate there. Neptune's moon Triton is believed to be a captured KBO. Pluto, a dwarf planet, is the largest known member of the Kuiper belt. Originally considered a planet, it is similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "Plutinos".
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs)
Known objects in the Kuiper belt, derived from data from the Minor Planet Center. Objects in the main belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known Trojan asteroids are yellow, while Jupiter's are pink. The scattered objects between the Sun and the Kuiper belt are known as centaurs. The scale is in astronomical units. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way.
The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of several wide-field survey telescopes such as Pan-STARRS and the future LSST, which should reveal many currently unknown KBOs. These surveys will provide data that will help determine answers to these questions. The Kuiper belt is believed to consist of planetesimals; fragments from the original protoplanetary disc around the Sun that failed to fully coalesce into planets and instead formed into smaller bodies, the largest less than 3000 km in diameter.
Modern computer simulations show the Kuiper belt to have been strongly influenced by Jupiter and Neptune, and also suggest that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are believed to have formed closer to Jupiter, and migrated outwards during the course of the Solar System's early evolution. Eventually, the orbits shifted to the point where Jupiter and Saturn existed in an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational pull from such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing Neptune's orbit to move outward into the primordial planetesimal disk, which sent the disk into temporary chaos. As Neptune traveled along this modified orbit, it excited and scattered many TNO planetesimals into higher and more eccentric orbits, depleting the primordial population. However, the present most popular model still fails to account for many of the characteristics of the distribution and, quoting one of the scientific articles, the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".
Simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter
