The Oort cloud is a vast spherical region of icy objects that surrounds our solar system in all directions. The Oort cloud is named for the Dutch astronomer Jan Oort who first proposed its existence. It’s occupied by countless comet-like objects that can be hundreds of feet in diameter, up to several miles in size. The Oort cloud lies far beyond Pluto and the Kuiper belt objects. Pluto and other Kuiper belt objects orbit the Sun between 30 and 50 times the distance of the Earth to the Sun. In contrast, the Oort cloud extends from 2,000 up to 100,000 times the Earth-Sun distance, at the very edge of the Sun’s gravitational influence. No one has ever actually seen the Oort cloud, it’s too far away and the objects that exist there are too small to be detected by current technology.
The existence of the Oort cloud has been inferred by calculating the orbital paths of the objects known as “long period” comets. These objects are different from the “short period” comets like Halley’s comet or the other “periodic” comets that circle the Sun every few years, up to periods of about 200 years. It may be possible to see some of these short period comets several times in one lifetime. In contrast, the long period comets visit the inner solar system from orbits that take them far out to the outer reaches of the solar system. These comets have orbits that are so deep in space that many of them only swing around the sun in periods that are thousands, or tens of thousands of years long. For us as observers, seeing an Oort cloud comet is a once-in-a-lifetime event.
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