The Oort cloud of comets surrounds the Solar System.

The Solar System is bigger than you think.

Before we talk about the Oort cloud, let's take a quick tour through the Solar System. The distances won't be in millions and billions of kilometers (or miles). They will be in astronomical units (AU) — 1 AU is the distance from the Earth to the Sun.

Starting at the Sun, we cross the inner Solar System. That takes us almost to Jupiter. Then we pass Jupiter, Saturn, Uranus and Neptune. Jupiter was at 5 AU, five times the distance from the Earth to the Sun. When we get to Neptune, we're 30 AU from the Sun. The Sun would look like a bright star from there.

Leaving Neptune, we enter the Kuiper belt. It's big! The far side is 50 AU where the scattered disk begins. The scattered disk ends at about 100 AU. (You can find out more about this region in the article "Kuiper Belt – Facts for Kids.")

But we still have a long journey to the Oort Cloud. No one really knows where it begins, but most astronomers suggest 10,000 AU. Not only is that a long way off, but it then stretches to 100,000 AU – or maybe up to twice that. This is trillions of miles, about a third of the way to the next star.

Comets often break up, so there must be a store of them somewhere in the distant outer Solar System.

We know that comets crash into planets. In July 1994 Jupiter's gravity broke up comet Shoemaker-Levy 9, and the pieces fell onto the planet. [Image: Hubble Space Telescope]

Many comets fall into the Sun. And whenever a comet goes around the Sun, it loses some of its material. Comets have many problems surviving, and yet there are still comets. Dutch astronomer Jan Oort realized this meant that there must be a reservoir (a storage place) of comets at the edge of the Solar System. The Oort cloud is named for him.

You can see a drawing showing what the Oort cloud might be like. The Oort cloud surrounds the Solar System. The drawing shows the cloud sliced like a cake so you can see the Solar System inside it.

Comets formed with the other Solar System bodies.

Planets, moons, asteroids and other bodies are made out of material left over from forming the Sun. Since the material was near the Sun, comets must have been closer to the Sun than they are now. The gravity of the giant planets pushed a lot of the comets into the outer Solar System, and kicked the rest of them into the Sun where they were destroyed.

The Sun's gravity has a very weak hold on the Oort cloud.

Gravity can work over very long distances, but it gets weaker and weaker as the distance gets longer. The Oort cloud is so far from the Sun that comets can also be affected by the gravity of other stars. Occasionally, a passing star disturbs the orbits of some of the comets, sending them into the inner Solar System.

Comets come from both the Kuiper belt and the Oort Cloud.

The orbits of short-period comets tend to go around the Sun and to the outer planets, returning in less than 200 years. They come from the Kuiper belt or the scattered disk. Long-period comets aren't so predictable, and arrive from all sorts of different directions. They have come from the Oort Cloud.

Some of our comets are missing.

There are probably more than a trillion comets in the Oort cloud with a total mass about five times that of the Earth. Astronomers think that there should be more of them. Although many comets come close to the Sun, many more have probably been knocked right out of the Solar System.