Science: Oort cloud

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The Oort cloud is a spherical cloud of comets which may lie nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane.

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 galaxy itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System. No confirmed direct observations of the Oort cloud have been made, but it is believed to be the source of some comets.

Sources: Wikipedia