Oort Cloud

The Oort Cloud (aka Öpik-Oort Cloud) is the name given to distant long-period comets, along with other such icy bodies at the same distance that are in less-eccentric orbits that don't bring them within our view or that lack the position/velocity combination of a solar orbit. This cloud is presumed to stretch outward from roughly 2,000 AU, which is far beyond the planets and the edge of the heliosphere, to a light-year or more from the Sun (one cited far edge is 200,000 AU, which is about 3 light-years, 3/4 the distance to Alpha Centauri). These distances are so far from the Sun that any encounter by the comet with another body (such as with a passing star; some stars are calculated to have passed closer to the Sun than Alpha Centauri's distance, and some are calculated to do so in the future) could significantly modify comet's orbit, potentially removing it from a solar orbit. Or such an encounter could shift a free-floating comet into a solar orbit. Such a set of objects is presumed to exist because the orbits of long-period comets have them spend so much time there (tens of thousands of years), and their orbits would be so easily changed, that observed orbits are unlikely to be unchanged over the age of the solar system (billions of years). Based upon the range of orbital inclinations of long-period comets, the Oort cloud is clearly spherical, in contrast to the Kuiper Belt which is generally concentrated around the ecliptic. One presumes other stars have such clouds though we don't have the direct evidence of observing their long-period comets; perhaps the logical far edge of the Oort cloud is to cover the distance over which a floating comet is more likely to be nudged into an orbit around the Sun than around another star.