Oort Cloud

The Oort cloud (pronounced /ˈɔrt/ ort, alternatively the Öpik-Oort cloud IPA: [ˈøpik]) is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.^[1] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest extrasolar star. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.^[

http://en.wikipedia.org/wiki/Oort_Cloud