The Oort clou (pronounced /ort/ ort, alternatively the Öpik-Oort Cloud (IPA: ['øpik])) is a spherical cloud of comets believed to lie roughly 50,000 AU, or nearly a light-year, from the Sun; this distance places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. 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 boundary of our Solar System.