This is an illustration of what the Oort cloud might be like.
Click on image for full size
Besides planets and moons, the solar system contains comets and asteroids. The place from which the comets come is thought to be a giant sphere surrounding the solar system. This sphere is called the Oort cloud. It is named after a scientist called Jan Oort who first thought of the idea in 1950. Thus comets
are said to come from the Oort cloud
The Oort cloud is very far away. In this picture, the solar system is buried deep within the cloud. An AU is the distance from the earth to the sun and is roughly equivalent to 100,000,000 miles. Mars is 1.5 AU from the sun, Jupiter is 5 AU from the sun, and Pluto is 39 AU from the sun. The Oort cloud is 100,000 AU away. So comets come from very far away indeed.
It is natural to ask "How did comets get so far away?" Astronomers are presently working to understand the answer to that question. This is one reason NASA studies comets.
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