The Oort Cloud is a spherical shell of icy bodies believed to surround the solar system at distances of thousands to a hundred thousand astronomical units. It has never been observed directly; its existence is inferred from long-period comets, which arrive from every direction rather than along the plane of the planets. Passing stars and the gravitational tide of the galaxy occasionally nudge one of its members inward. It marks the outer edge of the Sun's gravitational domain.