The Origin of CometsThe Oort Cloud
The origin of comets is still uncertain. They were once thought to have originated outside the solar system, but more recent theories suggest they were formed during the formation of the solar system and are permanent members of it. According to the storage-cloud hypothesis of J. H. Oort, a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 75,000–150,000 astronomical units (1 astronomical unit, or AU, being the mean distance from the earth to the sun). While the comets move very slowly in this huge storage cloud, a passing star may change their orbits enough to force some of them into the inner part of the solar system. The mechanism for the Oort cloud's creation, however, is unclear, and it has been suggested that the Oort cloud may include a significant amount of material that originated outside the solar system and was gravitationally captured by the sun.
In 1951, G. P. Kuiper, noting that Oort's cloud of comets did not adequately account for the population of short-period comets (those making complete orbits around the sun in less than 200 years), proposed the existence of a disk-shaped region of minor planets outside the orbit of Neptune, now called the Kuiper belt, as a source for such comets. The Kuiper belt acts as a reservoir for these in the same way that the Oort cloud acts as a reservoir for the long-period comets. This theory was validated in 1992 with the discovery of the first of the more than 70,000 so-called transneptunian objects, bodies more than 60 mi (100 km) in diameter in an orbit 30–50 AU from the sun. Astronomers regard Pluto not as a planet but rather as a member of the Kuiper belt. The discoveries of several Kuiper belt objects have led to this view. Eris, an object discovered in 2003 (and originally nicknamed Xena), has an elongated orbit that extends to roughly three times the distance of Pluto's, has a diameter (1,500 mi/2,400 km) slightly larger than that of Pluto, and has a moon; Quaoar is more than half the size of Pluto; and Ixion and Varuna are almost half the size of Pluto. 2003 VS2 (roughly a fourth the size of Pluto) and a number of other Kuiper belt objects, called plutinos, have an orbital synchrony with Neptune like that of Pluto (Neptune completes three orbits around the sun in the same time that Pluto and the plutinos complete two orbits).
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