# Wikijunior:Solar System/Oort Cloud

{{/Coolfacts\|= 1\|\|= 2\|Oort cloud
Facts:\|= 3\|\* The Oort cloud is as far from the Sun as you can go
without leaving the Solar System.

-   It is believed that most comets originate in the Oort cloud before
    \"falling\" toward the Sun.

}} The Oort cloud is a huge halo made of millions of comets that lies at
the outermost edge of the Solar System.

## What is the Oort Cloud?

Scientists say there is a distant group of objects, made of rock and
ice, that forms a cloud-like region surrounding our Solar System. It is
a cloud of comet-like objects orbiting far away from the Sun. Even
though the comets are very widely scattered from each other, there many
millions of them. The total mass of all these comets may be up to 100
times the mass of the Earth. The Oort Cloud is named after a Dutch
astronomer Jan Oort who took the
original idea, improved upon it and made it widely known.

!This diagram shows about how far away the Oort cloud might be compared
to the planets of the Solar System. Start in the upper left frame, then
follow the pictures clockwise. Each picture shows a bigger volume of
space.{width="350"}

As a comet makes several passes through the solar system, the Sun slowly
melts and vaporizes the ice and only little bits of solid debris are
left behind. But if the comets are all destroyed when they pass through
the system, then new comets will need to appear. Otherwise we would not
see any more comets. Jan Oort used the idea of the Oort cloud to explain
why new comets keep appearing.

## Where is the Oort Cloud?

If you can imagine the distance from the Earth to the Sun, then the
comets in the Oort cloud are 50,000 to 100,000 times further away! That
is 1,000 times further away from the Sun than is Pluto, and about one
fourth the distance to the nearest neighboring star---Proxima Centauri.
Light takes a year to travel from the Sun to the outer edge of the Oort
Cloud.

{{  }}

## How did the Oort Cloud start?

The Oort cloud objects may have started closer to the Sun during the
Solar System\'s formation. When they passed near the gas giants, the
gravity of those planets hurled the objects into very distant orbits.
The Oort cloud objects were sent in all directions, making the Oort
cloud ball-shaped instead of disk-shaped. The gravity of passing stars
also made the orbits of these objects more circular, and pulled them
further from the Sun. But sometimes the gravity of other far away stars
can send the objects hurtling back toward the Sun. These become the
comets.

## What objects are in the Oort Cloud?

An object named Sedna has been discovered that may belong to the Oort
Cloud (although it is actually between the Kuiper Belt and the Oort
Cloud.) It is from 1,180 to 1,800 km across. Its orbit stretches from 76
to 928 times further from the Sun than does the Earth\'s. Sedna orbits
the Sun about once every 11,250 Earth years. The last time Sedna was
where it is now in its orbit, Earth\'s last Ice Age was ending! Some
scientists think that Sedna should be included in the Kuiper Belt,
making the belt bigger.

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