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Beyond the Solar System - A solitary frozen comet in the Oort cloud; Oort cloud object; OCO; star perturbations; galactic tide - Space Art Illustration

The Oort Cloud

A mile-wide comet composed of ices and rock pursues a leisurely, million-year orbit around the Sun. At 20 thousand times Earth's distance from the Sun, the Sun is no brighter than the planet Venus in the evening sky on Earth. The temperature on the comet's surface is just a few degrees above absolute zero. (Comets develop their brilliant tails only when they enter the relative warmth of the Solar System itself.)

There is a region in deep space, far beyond the orbit of Pluto, but still under the gravitational influence of our sun, where it is believed swarms of comets orbit our solar system like bees around a hive. 

The comets in the Oort cloud (named after the Dutch astronomer who first proposed the cloud's existence in 1956) are leftovers from the formation of the Sun and planets of our solar system. Current estimates put the number of comets in the trillions, representing a total mass of about 40 Earths. While these are big numbers, this "cloud" is hardly crowded; individual comets are thought to be separated by an average of 100 million miles, about the same distance between the Sun and Earth.

In the far distant future--more precisely 1.4 million years from now--the nearby red dwarf star Gliese 710 will pass through the swarm of comets in the Oort cloud. While Gliese 710 will not come close enough to directly affect the Earth, it will likely knock some comets out of their orbits and possibly send a few down toward the Sun. 

Whenever comets enter the Solar System there is the risk that one or more may strike the Earth. It is thought that the Oort cloud has been jostled in this way before. 65 million years ago a close encounter with a larger star may have sent thousands of comets cascading toward the Sun. At least one of these comets struck the Earth and decimated 60% of all living species ending the reign of the dinosaurs.

Copyright Walter B. Myers. All rights reserved.

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