The Oort Cloud is a band of icy debris surrounding the entire Solar System from which long-period comets are thought to originate. Long-period comets are those that have orbital periods of thousands or even millions of years. The Oort cloud is believed to surround the Solar System at a distance of 100,000 astronomical units (100,000 times Earth's distance from the Sun), which is about halfway the distance to the nearest star to our Sun, proxima centauri.
In 1950, the Dutch astronomer Jan Oort proposed of a cloud of comets surrounding the Solar System.
Sometimes a passing star can cause a slight change in the icy outer bodies which causes them to come streaking into the inner solar system where we see them as comets.
The Oort Cloud consists of about 10 trillion comets. They have been here since the creation of the Solar System 4.6 billion years ago. The Cloud is 1.6 light years (7.6 million million km) across.
It takes a comet on the edge of the Sun. Comets cannot exist beyond the Oort Cloud because the Run's gravity is not strong enough to stop them being pulled away by a passing star.
The combined mass of all the comets in the Oort Cloud is equivalent to about three Earth's.
