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Oort cloud
A cloud of comets lying about 50,000 to 100,000 astronomical units from the sun.

The Oort cloud is postulated as the source of comets entering the solar system and is named after the Dutch astronomer Jan Oort, who theorized its existence in 1950.

The Oort Cloud is a spherical region extending for trillions of miles or kilometers that contains debris flung outward by the giant planets after they formed.

It is believed that comets that orbit the sun in periods of thousands or millions of years usually come from the Oort Cloud.

—Compiled from information presented in
Astronomy, The World Book Encyclopedia of Science;
World Book, Inc.; 2000; page 93.
This entry is located in the following unit: Astronomy and related astronomical terms (page 17)