The Oort Cloud is a hypothesised vast reservoir of icy small bodies surrounding the Solar System at enormous distances from the Sun. Unlike the Kuiper Belt, which is relatively flattened near the plane of planetary orbits, the Oort Cloud is expected to be approximately spherical because the bodies have been scattered onto highly inclined and often extremely elongated orbits by gravitational interactions.
Its exact outer boundary is uncertain, but commonly discussed estimates extend from thousands of astronomical units to perhaps tens or hundreds of thousands of astronomical units from the Sun. No spacecraft has directly observed an Oort Cloud object at its presumed native distance. Its existence is inferred primarily from the orbital properties of long period comets and from dynamical models of Solar System formation and evolution.
The Oort Cloud is named after Dutch astronomer Jan Oort, who developed a major theoretical explanation for the source of long period comets in the twentieth century. Long period comets have orbital periods ranging from hundreds to millions of years and can arrive from directions distributed across the sky rather than remaining concentrated near the ecliptic. Their nearly isotropic distribution is difficult to explain with a reservoir confined to the original planetary disk.
Oort proposed that cometary bodies were scattered outward by interactions with the giant planets and subsequently stored in distant gravitationally bound orbits. This idea developed into the modern Oort Cloud concept, although current models contain substantial refinements concerning formation, structure, dynamics and the contribution of external stellar and galactic influences.
The Oort Cloud is thought to have formed from planetesimals that originally occupied the region of the young Solar System closer to the giant planets. Gravitational encounters with Jupiter, Saturn, Uranus and Neptune could have increased the orbital semimajor axes of small bodies dramatically. Some scattered objects acquired very distant but still bound elliptical orbits.
Encounters with passing stars and the gravitational field of the Milky Way could then alter the orientations and perihelion distances of these distant objects, helping to randomise their orbital directions. The resulting population would not necessarily have formed directly at its present enormous distances. Instead, the Oort Cloud can be understood as a dynamically produced reservoir created through scattering and subsequent external perturbations.
The distinction between the inner and outer Oort Cloud is used in many dynamical discussions. The inner Oort Cloud is sometimes associated with objects whose orbits are more strongly bound to the Solar System and whose perihelia can be relatively stable against certain perturbations, while the outer Oort Cloud contains bodies more weakly bound to the Sun and therefore more susceptible to stellar and galactic gravitational effects. These are conceptual dynamical regions rather than sharply defined physical shells. The gravitational sphere of influence of the Sun also becomes less dominant at great distances, making the dynamics of Oort Cloud bodies sensitive to the Solar Systems motion through the Galactic environment.
Comets enter the observable inner Solar System when their orbits are perturbed so that their perihelia become sufficiently close to the Sun. Passing stars, molecular clouds, galactic tides and interactions within the distant Solar System can modify an objects orbit. A sufficiently strong perturbation can lower perihelion distance and place the comet on a trajectory toward the planetary region. Once solar heating becomes significant, volatile ices sublimate and create a coma, while dust and gas can form tails influenced by sunlight and the solar wind. The comet then becomes observable even though the reservoir from which it originated remains far beyond the reach of current spacecraft.
The Oort Cloud provides evidence about the early Solar System because its population records the gravitational scattering of primordial planetesimals. The detailed distribution of long period comet orbits can therefore constrain models of giant planet migration, stellar encounters and the early stellar environment of the Sun. The young Sun probably formed within a stellar association rather than in complete isolation, and close encounters with neighbouring stars may have influenced the outermost population.
However, the exact contribution of different formation pathways remains uncertain. Some distant objects may also have been captured from the Solar Systems original birth environment or from other systems under suitable dynamical conditions, although the quantitative importance of such processes remains an active research topic.
The Oort Cloud is difficult to observe directly because objects at such distances receive extremely little sunlight and appear extraordinarily faint. Even relatively large bodies would be difficult to detect unless they became active comets and moved inward. Consequently, estimates of its population depend heavily on modelling the observed long period comet flux and correcting for observational selection effects. The inferred total number of objects can therefore vary substantially between models. The absence of direct imaging of the cloud means that statements about its exact shape, density and population should be regarded as theoretical inferences rather than direct observations.
The Oort Cloud is distinct from the Kuiper Belt and from the scattered disk. The Kuiper Belt is a relatively flattened outer planetary system population beginning near Neptunes orbit, whereas the Oort Cloud is predicted to be much farther away and approximately spherical. The scattered disk consists of objects on dynamically excited trans Neptunian orbits that remain much closer to the planetary region than typical Oort Cloud bodies. The Oort Cloud is also not a solid shell or sharply bounded structure. It is expected to contain individual objects separated by enormous distances, with a broad and uncertain distribution of orbital elements.
