Oort Cloud formation and evolution, illustration. At upper left is the primordial comet cloud with the orbits (red ellipses) of the four giant planets of the outer solar system, and the Sun at centre. At upper right, the planets spiral outwards and comets are ejected to the Oort Cloud. At centre left, comets merge to former larger objects. At centre right the chaotic orbits of Uranus and Neptune results in the ejection of Kuiper Belt Objects to form a region known as the 'scattered disc'. At bottom, the Kuiper Belt (white ring) is shown within the spherical Oort Cloud, with short-period and long-period comet orbits shown in yellow, with planetary orbits in red.