The Oort cloud is a hypothetical swarm of potentially quadrillions of icy planetesimals, extending in a spherical shell from around 2,000 to 5,000 astronomical units (AU) to around 50,000 to 200,000 AU from the Sun. The outer edge of the Oort cloud effectively marks the furthest limit of our Solar System.

The Oort cloud is named after Jan Oort, a Dutch astronomer who proposed its existence to explain the origin of long-period comets. The objects that make up the Oort cloud are believed to have formed from the Sun’s primordial protoplanetary disk. This would mean that the majority would be expected to have a similar composition to Kuiper belt objects. This matches the observations that long and short-period comets seem to share similar compositions, with the latter believed to originate mainly from the Kuiper belt.

No objects from the Oort cloud have yet been positively identified, due to their size and extreme distances from Earth. However, the trans-Neptunian object Sedna has an extremely elliptical orbit that is though to be too far out to have been caused by the gravitational influence of Neptune. It is possible that Sedna is an inner member of the Oort cloud.

Voyager 1 is the furthest of our space probes from Earth, which crossed the heliopause, in 2012. This marks one of the boundaries of the solar wind in the outer regions of the Solar System. However, Voyager 1 will take an estimated 300 years to reach the inner edge of the Oort cloud and a further 30,000 years to reach the other side.