The Oort Cloud is a theoretical spherical region of comets found beyond the Kuiper belt and the scattered disk (now known to exist), and stretching as much as an entire light-year away from the Sun. These comets, if they exist, would primarily consist of ice, and would have been ejected from the solar system thanks to the gravitational effects of the gas giants (Jupiter, Saturn, Uranus, and Neptune). The Oort Cloud is currently further divided into two broad sectors, the inner Hills Cloud and the outer Oort Cloud. Only a handful of objects yet seen are thought to be potential Oort Cloud objects.
- Location and Composition -
Beyond the orbit of Neptune likes a population of small objects called the Kuiper Belt; it is now believed that Pluto (recently demoted to dwarf planet status) is properly considered as an unusually large Kuiper Belt Object. Beyond the Kuiper Belt lies an even more scattered population of obects known as the scattered disk, to which another large dwarf planet, Eris, may belong. (Eris is larger than Pluto, and it was Eris's discovery which sparked the Pluto planet debate several years ago.) However, beyond the scattered disk, while the Sun's light and gravity are growing weak there is no theoretical reason why no further objects can exist.
In particular, our knowledge of comets suggests that a population of objects lies outside of the Kuiper Belt and the scattered disk. Although there might not be enough matter this far out for objects to have formed on their own during the early solar system, a very large number of comets would have been ejected from the system through the gravitational influence of the gas giants shortly after they formed. Some comets pass through the inner solar system on orbital paths that last thousands of years, which brings them far enough away from the solar system that they cannot be properly classified as Kuiper Belt comets.
If the Oort Cloud does exist, it likely contains several trillion objects ranging in distance from about two thousand times as far from the Sun as Earth (2000 Astronomical Units, or AU) as far as 50,000 AU or even twice that much. There would be a gradual but continuous cycling of cometary material moving outward from the Kuiper Belt through the Hills Cloud into the Oort Cloud as it was ejected from the solar system, and other material being tugged the other direction and moving from the Oort Cloud back inward again. This far out, the vast majority of matter would surely be in ice form, although this does not rule out rocky bodies as well, especially if the Cloud is made up of material which originated much closer to the Sun than its current location.
- History of Theory and Observation -
The theory of the Oort Cloud originated in the 1930s with Ernest Opik of Estonia, who was trying to devise a theoretical explanation for the unusual comets mentioned above (known as long-period comets, because of the extremely long period of their orbits). Opik's theory was revived after World War II by Jan Oort of the Netherlands, after which the cloud is now named. Oort was also interested in comets, but he was attempting to explain a different problem.
Oort's problem was as follows. Comets are composed in great part of icy material, some of which is boiled away during the part of its orbit when the comet is close to the Sun. (It is the trademark smudge, tail, or "coma," of debris and gas which make comets visible in the night sky from Earth.) Moreover, as they make their passes through the solar system, their orbits must gradually be altered by the gravitational influence of the planets they pass by. Eventually, an orbiting body with an unstable orbit will either be pulled directly into the Sun (or Jupiter), captured by a large planet as a moon, or will be thrown out of the solar system entirely. Yet comets still exist, and still throw off trails of gas as they approach the Sun. It followed, Oort reasoned, that these comets had not actually been following their long elliptical orbits through the solar system for very long. They must have spent most of the history of the solar system harboured in some sort of distant population of comets, out of which the gravity of the Sun or the outer planets occasionally pulled some out. We now refer to this theoretical cometary population as the Oort Cloud, in honour of Jans Oort.
Theorizing the Oort Cloud and actually seeing it, however, are two very different problems. The cloud is far enough away that even large objects are difficult to see - and, according to the theory, there are extremely few large objects in the cloud. Leaving aside Opik's original assumption that most or all comets with very long orbital periods belong to the Oort Cloud, the number of likely members of the Oort Cloud population sighted to date can be counted on the fingers of one hand. One of the candidates is an actual dwarf planet, Sedna. Sedna's orbit (which ranges from about 75 AU at its closest to just under 1000 AU at its most distant) is closer than the theoretical limits of the Oort Cloud. At the same time, it is far enough away that astronomers believe it may not be affected by Neptune's gravitational influence, even at its closest point (75 AU). If true, then by definition it could not belong to the Kuiper Belt or the scattered disk, and therefore might end up being classified as an Oort cloud object by default. All of the candidate objects were discovered within the past ten years, and more will probably follow in subsequent years.