Early in the history of our planet, many comets may have collided with earth provided the much needed water to support life. Current theory holds that the solar system formed about 4.6 billion years ago from a large dust cloud left from old stars. The cloud collapsed to form the planets but the most distant dust collected into many small icy rocks that are more than 1,000 times farther than Pluto from our sun. This region of space about our solar system is called The Oort Cloud of Comets. These small icy rocks are occasionally deflected by other larger bodies that pass near the Oort Cloud throwing them back into the solar system at highly elliptical orbits.
Comets can suddenly brighten or fade from view in a matter of hours.
They can loose their tail or develop multiple tails or break apart and fall into the sun. Sometimes they can even split into two or more pieces, so that, through a telescope, several comets can be seen moving together across the sky. Once brought into the inner solar system, comets life span shorten greatly. After just a few orbits about the sun, the ice is melted away and all that remains is stellar dust or rock. Comets are also cause many of earth's well known meteor showers.