The Kuiper Belt and Oort Cloud
The Kuiper belt
The Kuiper belt is a belt of asteroids and comets that extends from the orbit of Neptune, at about 30 AU, to over 55 AU (one AU is the distance between the Earth and the Sun). Short-period comets, however, can travel up to 100 AU from the Sun, into a region known as the scattered disc. Short-period comets are comets that reappear within 200 years, like Halley’s Comet, which has a period of about 76 years.
The Kuiper belt was theorised shortly after the discovery of Pluto, and is named after Dutch-American astronomer Gerard Kuiper, who discussed the idea in 1951. The first Kuiper belt objects were detected by British astronomer David Jewitt, and American astronomer Jane Luu, in 1992. Over 70,000 objects, over 100 km in diameter, have since been found.
The Kuiper belt contains at least three dwarf planets, Pluto, Haumea, and Makemake.
Pluto, image from New Horizons. Image credit: Modified from NASA/JHUAPL/SWRI.
Pluto is mostly composed of rock and ice. It is five times less massive than the Earth's Moon and has an eccentric orbit, which sometimes takes it closer to the Sun than Neptune. Pluto is orbited by a number of moons including Nix, Hydra, and Charon. Charon is the largest, with a diameter over half the size of Pluto's.
The Pioneer and Voyager probes all passed through the Kuiper belt but did not get close to any large objects. The first spacecraft sent to explore the Kuiper belt, New Horizons, was launched by NASA in 2006, and arrived at Pluto on 14th July 2015. Data from the New Horizons space probe has shown that Pluto is geologically active, and contains ice plains, flowing ice, and mountains.
Objects beyond the orbit of Neptune and the Earth, to scale. Image credit: NASA/Lexicon.
The Oort cloud
The Oort cloud is a spherical cloud of comets, which orbits between about 5000 AU and 100,000 AU. This is over 1.5 light years, 2000 times further than the edge of the Kuiper belt, and a third of the distance to the closest extra-solar star, Proxima Centauri.
Oort cloud objects are thought to be composed of frozen water, ammonia, carbon monoxide, hydrogen cyanide, ethane, and methane. They can be affected by the gravitational force of nearby stars and this sometimes sends them towards the centre of the Solar System. In 1932, Estonian astronomer Ernst Öpik suggested that long-period comets, such as Hale-Bopp, may originate from the Oort cloud and this idea was extended by Dutch astronomer Jan Oort in 1950. The Oort cloud is so far away that the Pioneer and Voyager probes will not pass into it for hundreds of years.
Artists' impression of the Kuiper belt and Oort cloud. Image credit: NASA/JPL.
There have been a number of missions to comets, with the European Space Agency's (ESA's) Rosetta spacecraft currently in orbit around the comet Churyumov-Gerasimenko. It is due to become the first spacecraft to land on a comet in November 2014.