The Kuiper Belt is a flat ring of icy small bodies orbiting the Sun beyond the orbit of Neptune. Named after the Dutch-American astronomer Gerard P. Kuiper, this belt consists of hundreds of millions of objects whose orbits are close to the plane of the solar system. These objects are thought to be remnants from the formation of the outer planets.

The Kuiper Belt is believed to be the source of most observed short-period comets, especially those that orbit the Sun in less than 20 years, as well as icy Centaur objects that have orbits within the region of the giant planets. Although its existence was theorized for decades, the Kuiper Belt was not detected until the 1990s, when large telescopes and sensitive light detectors became available.

On average, these objects orbit the Sun at a distance greater than Neptune's average orbital distance (about 30 astronomical units (AU), or 4.5 billion kilometers). The outer edge of the Kuiper Belt is less clearly defined, but it nominally excludes objects that come closer than 47.2 AU (7.1 billion kilometers) to the Sun, which is the location of the 2:1 Neptune resonance, where an object completes one orbit for every two orbits of Neptune. The Kuiper Belt includes large bodies such as Eris, Pluto, Makemake, Haumea, and Quaoar, as well as potentially millions of smaller objects.

^{The position of the planets and the Kuiper Belt in the solar system [2]}

Discovery of the Kuiper Belt

Irish astronomer Kenneth E. Edgeworth suggested in 1943 that the distribution of small bodies in the solar system was not limited to Pluto's current distance. Kuiper developed a stronger argument in 1951. Based on an analysis of the mass distribution of objects that formed the planets, Kuiper showed that there must be a large number of small icy remnants (active comet nuclei) beyond Neptune.

Dutch astronomer Jan Oort proposed the existence of a much more distant, spherical reservoir of icy objects, now called the Oort Cloud, which continuously replenishes comets. His proposal explained the origin of long-period comets (those with periods longer than 200 years). However, Kuiper pointed out that very short-period comets (20 years or less), which orbit in the same direction and plane as the planets, required a closer, flatter source. This explanation, clearly restated by American astronomer Martin Duncan and his colleagues in 1988, remained the best argument until the Kuiper Belt was directly detected.

^{Objects found in the Kuiper Belt [3]}

The Oort Cloud

The Oort Cloud has never been observed, but it is thought to be a spherical distribution of icy objects, like comets, orbiting the Sun at distances between 3,000 and 100,000 AU. It is also believed to be the origin of most long-period comets in the solar system. The objects in the Oort Cloud likely formed closer to the Sun, near the current orbits of Uranus and Neptune, and were later pushed to their current positions by gravitational interactions with the planets.

Astronomers theorize that the Oort Cloud has approximately 10^12 to 10^13 members, with a total mass of about 100 Earth masses. Objects within the Kuiper Belt are influenced by the gravity of the planets. Farther out, between 50 and 2,000 AU, is a region of the Oort Cloud where objects are not affected by the planets. Between 2,000 and 15,000 AU, objects in the cloud are influenced by galactic tidal forces, and in the outer Oort Cloud, between 15,000 and 100,000 AU, objects are affected by the gravity of other stars. Beyond the Oort Cloud, the Sun's gravity is not strong enough to keep objects in orbit.

^{The location of the Kuiper Belt and the estimated position of the Oort Cloud [1]}

^{The position of the Kuiper Belt in the solar system [4]}