Oort cloud (noun, “OR-t cloud”)

This is a shell of ice and rocks that surrounds our solar system and can be found out beyond Neptune and Pluto. Comets that take more than 200 years to orbit the sun are thought to come from the Oort cloud.

The Oort cloud begins about 750 million kilometers (465 million miles) away from the sun. (The Earth orbits some 146 million kilometers, or 92 million miles, from our star.) The outer edge of the Oort cloud is nearly 15 trillion kilometers (9.2 trillion miles) away. This makes for a very thick cloud. The Voyager I spacecraft, which left Earth in 1977, has been making its way out of the solar system ever since. Voyager has long left Neptune far behind. But it won’t reach the beginning of the Oort cloud for another 300 years. And it may take another 30,000 years to come out the other side.

But for all that it’s big and thick, scientists have never actually seen the Oort Cloud. The astronomer Jan Oort predicted that this cloud of bodies existed, and it now bears his name. But so far, there have been no missions to detect, let alone visit, the shell.

In a sentence

In 2013, a comet from the Oort Cloud buzzed past the sun.

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