1. 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[2a].
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 Vietnamese-American astronomer Jane Luu, in 1992. The Kuiper belt is now thought to contain hundreds of thousands of objects over 100 km in diameter[2b], and over 1000 of these have been catalogued.
The Kuiper Belt and scattered disc contain at least four dwarf planets: Pluto, Eris, Haumea, and Makemake[2c]. The closest is Pluto, which was discovered by American astronomer Clyde Tombaugh and his colleagues in 1930, and demoted from planetary status in 2006.
Pluto, image from New Horizons. Image credit: NASA/JHUAPL/SWRI/Public domain.
Charon, image from New Horizons. Image credit: NASA/JHUAPL/SWRI/Public domain.
Pluto is mostly composed of rock and ice. Its mass is about one-sixth of the Earth's Moon, and it has an eccentric orbit, which sometimes takes it closer to the Sun than Neptune. Pluto has at least five moons, including Nix, Hydra, and Charon. Charon is the largest, with a diameter over half the size of Pluto's.
The Earth, and objects beyond the orbit of Neptune, sizes are to scale. Image credit: NASA/Lexicon/Public domain.
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, NASA's New Horizons, was launched in 2006, and arrived at Pluto in 2015. Data from the New Horizons space probe has shown that Pluto is geologically active, and contains ice plains, flowing ice, and mountains.
There have been a number of missions to both short and long-period comets, including the European Space Agency's (ESA's) Rosetta mission, which involved landing on the short-period comet Churyumov-Gerasimenko in 2014.
2. The Oort cloud ↑
The Oort Cloud is a spherical cloud of comets that orbit 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 extrasolar star, Proxima Centauri.
Image credit: NASA/Public domain.
Oort Cloud objects are thought to be composed of frozen water, ammonia, carbon monoxide, hydrogen cyanide, 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, which has a period of over 2500 years, may originate from the Oort Cloud. Dutch astronomer Jan Oort extended this idea in 1950.
There have been no missions to long-period comets, and the Oort Cloud is so far away that the Pioneer, Voyager, and New Horizons probes will not pass into it for hundreds of years.