Although no direct observations have been made of such a cloud, it is believed to be the source of most or all comets entering the inner solar system (some short-period comets may come from the Kuiper belt), based on observations of the orbits of comets.
The Oort cloud was proposed in 1950 by Dutch astronomer Jan Oort to explain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets we observe had existed since the origin of the solar system, all would have been destroyed by now.
According to the theory, the Oort cloud contains millions of comet nuclei, which are stable because the sun’s radiation is weak at their distance. The cloud provides a continual supply of new comets, replacing those that are destroyed.
The Oort cloud is a remnant of the original nebula that collapsed to form the sun and planets five billion years ago, and is loosely bound to the solar system.
It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars’ Oort clouds may sometimes overlap, causing the occasional intrusion of a comet into the inner solar system.
See also the Kuiper belt, a belt of cometary material that lies between Neptune’s orbit and the Oort.