The Oort Cloud is the massive spherical cloud that encases our entire Solar System. It is thought to extend from 5,000 AU (an AU is equal to the distance from the Sun to the Earth), out to about 50,000 AU, which is nearly a light year. It's composed of many trillions of objects larger than 1 km across, plus countless more of smaller sizes. The vast majority of comets originate here, slowly drifting through space until they are somehow knocked out of orbit and begin to fall towards the Sun.