Oort Cloud:
The Oort Cloud is an immense spherical cloud surrounding our Solar System. Extending about 30 trillion kilometers (18 trillion miles) from the Sun, it was first proposed in 1950 by Dutch astronomer Jan Oort. The vast distance of the Oort cloud is considered to be the outer edge of the Solar System - where the Sun's orb of physical and gravitational influence ends.
The Oort Cloud contains billions of icy bodies in solar orbit. Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as a long-period comet. These comets have very large orbits and are observed in the inner solar system only once. In contrast, short-period comets take less than 200 years to orbit the Sun and they travel along the plane in which most of the planets orbit. They come from a region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper, who proposed its existence in 1951.
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