Word of the Day -- Oort Cloud

The Oort cloud, named after Dutch astronomer Jan Oort and Estonian astronomer Ernst Öpik, is a spherical cloud of predominantly icy planetesimals believed to surround the Sun at a distance of up to around 100,000 AU (2 ly). This places it at half of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.

The Oort cloud is thought to comprise two regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud.

Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution. Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.