Shell of small bodies and dust enveloping the solar system at a distance of approximately 1.5 light years. It's believed to be made up of remnants from the formation of the solar system and the origin of most comets, possibly of spare "leftover" planetesimals ejected by gravitational forces during the formation of the major planets under the same principles that helped send Pioneer and Mariner space probes out of the solar system. Named after Dutch astronomer Jan Hendrik Oort who hypothesized its existence after observing the high eccentricity of comets' orbits.
The cloud itself has not been seen or measured but the hypothesis of its existence is generally accepted as fact by most astronomers. Its exact composition, density and size remain uncalculated and with current technology incalculable and unobservable. The best educated guess is that it contains 10-12 billion comet-sized objects. Unlike all other parts of the solar system, the Oort cloud is spherical and not flat in line with the Sun's orbital plane.