What is the Oort Cloud? - Definition, Location & Facts

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what the Oort cloud is, how it formed as part of the solar system's development, where it's located, and discuss its properties and effects. A short quiz will follow.

A Big Mess

Have you ever seen a toddler try to build something? Perhaps a gigantic tower, or a papier-mâché Teletubby? The end result is always a colossal mess all over the room.

Well, the forces that created the universe have a lot in common with that toddler. In the center of our solar system, we have our nicely compact Sun, surrounded neatly by each of the eight planets (NOT nine! -- Pluto is no longer considered a planet). But as you go further out, things start to get... messy.

The Oort cloud is a spherically shaped area filled with random bits of icy chunks and planetesimals that didn't make it into the Sun or main planets. And it's huge!

The Kuiper Belt and Oort Cloud
The Kuiper Belt and Oort cloud

How Did It Get There?

In the early Solar System, it was pretty chaotic. We've talked about one toddler... now try to imagine thousands of toddlers running around a playroom. There would be collisions, injuries and more than a few tears. It was the same as the Solar System formed. Objects were hitting each other all the time, until gravity eventually pulled them together enough to form the Sun and planets. Most of the material in the area formed the Sun, and it was only perhaps 1% of the material that was left over to form planets and other smaller rocks.

Most of the material, especially rock, coalesced into a disk shape. Have you noticed how a lot of the things in the universe are that shape? Most of the planets in our Solar System revolve around the sun in a flat disk, almost every galaxy is a spiral disk, and even our own planet Earth isn't a perfect sphere -- it's flattened slightly and bulges at the equator. This is just a thing that gravity tends to do to spinning objects. There will always be a direction that a cloud of rock and gas is spinning more in than the others, and this causes the objects to slowly collapse to a disk that spins in that direction.

But the Oort cloud is spherical, so how did that happen? While the topic is open for debate, most scientists believe that the Oort cloud is made up of objects that, through collisions, found themselves on wild elliptical orbits that they couldn't get out of. Comets are an example of the types of objects that come from the Oort cloud, but they rarely stay in our Solar System -- their orbits are wild enough that they just pass through periodically.

Orbiting Comet
Orbiting Comet

Although the Oort cloud is spherical overall, it does contain an inner cloud that is more disk shaped, called the Hills cloud.

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