hypothetical spherical region of the solar system
The Oort cloud, sometimes called the Öpik–Oort cloud is a hypothetical spherical region of the Solar System that consists of billions of tiny planetessimals surrounding the Sun at 2000 - 200 000 AU. It is also where long-period comets come from. The cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was named. Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.
Despite not being prooven, astronomers think this is the source for all Halley-type and long-period comets, also many centaurs and comets of Jupiter's family.
Short period comets can also come from the Oort Cloud, and not only Scattered disc.
The Outer Oort Cloud is the border of the Solar System, and can easily be affected by flying-by stars and galactical tidal forces. These two things cause comets to invade the inner Solar System.
Between 2004 and 2012 we discovered five Oort Cloud objects: Sedna, Biden, 2000 CR105, 2006 SQ372 and 2008 KV42. Since 2012 the amount of found Oort Cloud objects increased fast, for example C/2014 UN271 is a comet from the Oort Cloud. A lot were found thanks to the search for Planet X, for example The Goblin.
Some theories suggest the existence of an undiscovered planet or even several planets in the Oort Cloud: Planet Nine or Tyche, for example. Another theory suggests the existence of a second star in our Solar System in the outer Oort Cloud, Nemesis.