Oort cloud

The Oort cloud, (IPA: BrE /ɔːt klaʊd/, AmE /ɔɹt klaʊd/), alternatively termed the Öpik-Oort Cloud (/'øpik/-, like /'epik/ with a rounded /e/), is a postulated spherical cloud of comets situated about 50,000 to 100,000 AU from the Sun. This is approximately 2000 times the distance from the Sun to Pluto or roughly one light year, almost a quarter of the distance from the Sun to Proxima Centauri, the star nearest the Sun.
The Oort cloud would have its inner disk at the ecliptic from the Kuiper belt. Although no confirmed direct observations have been made of such a cloud, astronomers believe it 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 direct observations of the orbits of comets.
There is also a theory of a denser, inner part of the Oort cloud coined the Hills cloud; it would have a well-defined outer boundary at 20-30 000 AU, a less well defined inner boundary at 50 to 3000 AU, and would be about 10 to 100 times denser than the remainder.
In 1932 Ernst Öpik , an Estonian astronomer, proposed that comets originate in an orbiting cloud situated at the outermost edge of the solar system. In 1950 the idea was revived and proposed by Dutch astronomer Jan Hendrick Oort to explain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets we observe had really existed for billions of years (since the generally accepted origin of the solar system), all would have been destroyed by now. According to the hypothesis, the Oort cloud contains millions of comet nuclei, which are stable because the sun's radiation is very weak at their distance. The cloud provides a continual supply of new comets, replacing those that are destroyed. It is believed that if the Oort cloud exists and supplies comets, in order for it to supply the necessary volume of comets, the total mass of comets in the Oort cloud must be many times that of Earth. Estimates range between 5 and 100 Earth masses.