Oort Cloud

Oort Cloud

Oort Cloud, vast, nearly spherical swarm of comets and icy planetesimals (small planets) dating from the origin of the solar system and orbiting the Sun in all directions. The existence of the cloud is hypothetical. It has never been directly observed. Dutch astronomer Jan Hendrik Oort postulated its existence in 1950 and proposed that it was the source of long-period comets (comets that take 200 years or more to orbit the Sun). Astronomers estimate that more than 100 billion comets exist in the Oort cloud at a distance from the Sun of 20,000 to 200,000 astronomical units (AU). An AU is the mean distance between the Earth and the Sun, or about 150 million km (93 million mi).

The shape of the cloud’s outer boundary resembles a slightly flattened ball. However, closer to the Sun, at around 100,000 AU, the structure is more spherical. There, it is shaped by the gravitation of passing stars, interstellar clouds, and the gravitational field of the Milky Way Galaxy. The latter is believed to cause a roughly 30-million-year variation within the overall 200-million-year period that it takes for the solar system to make a complete orbit of the Milky Way. This cycle of gravitational perturbation peaks as the Sun passes through the galactic midplane, causing comets from the outer Oort cloud to be deflected into the inner solar system.

Astronomers theorize that the number of comets per unit volume increases steeply toward the Sun, culminating in a dense inner core that contains most of the cloud’s total mass. The inner core replenishes the outer reservoir as comets are lost due to stellar and molecular-cloud perturbations over the age of the solar system. Theories suggest that the part even closer to the Sun extends inward as a flattened, disklike structure, and eventually overlaps the Kuiper Belt, a doughnut-shaped ring of comets and planetesimals lying just beyond Neptune.

Most current ideas about the origin of the cloud assume that it was formed at the same time as the solar system, some 4.6 billion years ago. The gravitational influence of the planets scattered comets, asteroids, and planetesimals toward the outer regions of the solar system. Astronomers believe the Oort cloud contains a sample of the primordial material that formed the planets. It almost certainly includes objects that are smaller than known comets and others that are larger, with diameters ranging from a few meters to many hundreds of kilometers. The precise nature of most of these objects is unknown.

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.