Oort cloud - (n.)

a vast collection of bodies made up of rock and ice that orbit the Sun at a distance starting in the region beyond the orbit of Pluto and extending out to nearly 1.5 light-years or 50,000 A. U. Unlike the bodies of the Solar System, which orbit the Sun in roughly the same plane, the Oort cloud objects form a vast sphere around the Sun. It is estimated that billions of objects exist in this region, and there is evidence that this is the place where most comets originate.