The Oort Cloud

Beyond the Kuiper Belt lies the Oort cloud, an immense, roughly spherical cloud of icy small bodies that are inferred to revolve around the Sun at distances typically more than 1,000 times that of the orbit of Neptune, the outermost known major planet. Named for Jan Oort, who demonstrated its existence, the Oort cloud comprises objects that are less than 100 km (60 miles) in diameter and that number perhaps in the trillions, with an estimated total mass 10-100 times that of Earth. Although too distant to be seen directly, it is believed to be the source of most of the historically observed long-period comets—those that take more than 200 years (and usually much longer) to orbit the Sun.

The Estonian astronomer Ernest J. Opik in 1932 suggested the possible presence of a distant reservoir of comets, arguing that, because comets burn out relatively quickly from their passages through the inner solar system, there must exist a source of "fresh" comets, which steadily replenishes the comet supply. Although these comets have never been in the inner solar system before, they are difficult to distinguish from older long-period comets because, by the time they are first observed, their orbits already have been gravitationally perturbed by the outer planets.

In 1950, Oort showed by statistical arguments that a steady flux of a few "new" comets are observed per year (those that had never been through the solar system before). This flux comes from the fringe of the Oort cloud. He identified it by looking at the distribution of the original values of the total energies of cometary orbits. These energies are in proportion to a-1, with a being the semimajor axis of the cometary orbit. The original value of a refers to the orbit when the comet was still outside of the solar system, as opposed to the osculating orbit, which refers to the arc observed from Earth after it has been modified by the perturbations of the giant planets. Passages through the solar system produce a rather wide diffusion in orbital energies (in a _1). In 1950 Oort accounted for only 19 accurate original orbits of long-period comets. The fact that 10 of the 19 orbits were concentrated in a very narrow range of a'1 established that most of them had never been through this diffusion process due to the planets. The mean value of a for these new comets suggested the distance they were coming from—about 105 AU. This distance is also the place where perturbations resulting from the passage of nearby stars begin to be felt. The distance coincidence suggested to Oort that stellar perturbations were the mechanism by which comets were sent into the planetary system.

Subsequently, using a much larger number of observed orbits, the American astronomer Brian Marsden calculated that the part of the Oort cloud where new comets originate—the more distant part of the cloud—is between 40,000 and 50,000 AU from the Sun. At such distances, the orbits of the tiny icy bodies can be disrupted and sent inward by either of two processes: the occasional close passage of a star or giant interstellar molecular cloud near the solar system or the gravitational forces, called disk tides, exerted by the mass of the Galaxy's disk. Although the inner part of the Oort cloud, which is thought to begin at about 20,000 AU, does not supply comets, its existence and large mass are predicted by the theory of the origin of the solar system. The Oort cloud must have been created from icy planetesimals that originally accreted in the outer part of the protoplanetary disk and were then scattered far away by the gravity of the incipient giant planets. How far the Oort cloud extends into space is not known, although Marsden's results suggest that it is almost empty beyond 50,000 AU, which is about one-fifth of the distance to the nearest star.

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