Kuiper belt refers to the disc-shaped region beyond Neptune, named for astronomer Gerard Kuiper. It is presumed to be remnants from the formation of the solar system about 4.6 billion years ago.The Kuiper Belt extends from about 30 to 55 AU (One astronomical unit, or AU, is the mean distance of Earth from the Sun: about 150 million kilometers or 93 million miles.) and is probably populated with hundreds of thousands of icy bodies larger than 100 kilometers (62 miles) across and an estimated trillion or more comets.
Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how brightness relates to size. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.
In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the Sun at an extreme distance, in one of the coldest known regions of our solar system. The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the Sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU - instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the Sun. Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.
Classical KBOs are those having orbits with modest eccentricities and semimajor axes roughly in the 40 to 47 AU range. They are defined as a group by their long-term stability - they simply never approach Neptune closely enough to have been ejected over the age of the Solar system.
The name "Classical" was coined by me to reflect the fact that the near-circular orbits of these bodies are, amongst the known KBOs, the closest to expectations from dynamical models existing when the Kuiper belt was discovered. These models predicted that the Kuiper belt would be thin (it is very thick) and the orbits of KBOs would be nearly circular (most are highly elliptical). The "Classical" label means that these objects are the closest to this 'classical' expectation.
Objects in eccentric orbits (up to 0.8) having perihelia beyond 30 AU. They also tend to be inclined to the ecliptic by several 10s of degrees. Their defining characteristic is that gravitational perturbations from Neptune, at perihelion, seem sufficient to have pumped up the eccentricities to the observed large values (hence the term "scattered"). They are not protected from Neptune, in general, and this is a population that appears to be leaking away over the age of the Solar system.
They are also known as "scattered disk objects", although this is a poor term because their large inclinations do not much resemble a disk.
Pluto is the largest known member of the Kuiper belt. Originally considered a planet, Pluto's position as part of the Kuiper belt has caused it to be redefined as a "dwarf planet". It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "Plutinos". In Pluto's honour, the four currently accepted dwarf planets beyond Neptune's orbit are called "plutoids".
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).
