Origins
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 3,000 kilometres (1,900 mi) 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 their present positions, 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. Scattering of planetesimals early in the Solar System's history would have led to migration of the orbits of the giant planets; Saturn, Uranus and Neptune drifted outwards while Jupiter drifted inwards. Eventually, the orbits shifted to the point where Jupiter and Saturn reached an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational repercussions of such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing Neptune's orbit to become more eccentric and move outward into the primordial planetesimal disk, which sent the disk into temporary chaos. As Neptune's orbit expanded, it excited and scattered many TNO planetesimals into higher and more eccentric orbits. Many more were scattered inward, often to be scattered again and in some cases ejected by Jupiter. The process is thought to have reduced the primordial Kuiper belt population by 99% or more, and to have shifted the distribution of the surviving members outward.
However, this currently most popular model, the "Nice model", still fails to account for some 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". The model predicts a higher average eccentricity in classical KBO orbits than is observed (0.10–0.13 versus 0.07). The frequency of paired objects, many of which are far apart and loosely bound, also poses a problem for the model.
Read more about this topic: Kuiper Belt
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