Oort cloud
The Oort cloud (named after the astronomer Jan Oort, who first proposed its existence) is a spherical cloud of comets surrounding the Solar system. Its exact size is unknown, but it probably extends for two or more light years. The limiting factor is that it cannot extend to the vicinity of neighbouring stars, or their gravity would disrupt it.
Within the cloud, comets must be separated by typically tens of millions of kilometres. As they are weakly bound to the sun, especially those near the edge of the cloud, any passing stars and other forces can easily change their orbits, sending them into the inner solar system or out to interstellar space. The cloud is believed to have a relatively dense core that lies near the ecliptic plane and gradually replenishes the outer boundaries, creating a steady state. One sixth of an estimated six million million icy objects or comets are in the outer region with the remainder in the relatively dense core.
As well as stellar perturbations where a neighbouring star's Oort cloud passes through or close to the Sun's Oort cloud, other forces include the influences of giant molecular clouds and tidal forces. A giant molecular cloud is far more massive than the Sun. It is an accumulation of cold hydrogen that is the birthplace of stars and solar systems. These are infrequently encountered, about every 300-500 million years, but when they are encountered, they can violently redistribute comets within the Oort cloud, causing a shower of comets in the inner Solar system.