Also known as the Öpik–Oort cloud, after Dutch and Estonian astronomers Jan Oort and Ernst Öpik, the cloud is a spherical swarm of planetesimals composed mainly of various ices (notably water, methane, and ammonia ice) which are presumed to surround the Sun out to a distance of around 100,000 astronomical units (app. 2 light years), although some estimates put this distance as high as 200,000 AU.
The outer edge of the this spherical cloud defines the furthest extent of the Sun’s tidal influence, and thus the limit of the solar system; to put this in perspective, the Oort cloud extends to half the distance to Proxima Centauri, the Sun’s closest neighbor. In structure, the Oort cloud is thought to have two distinct regions, an outer, spherical shell that resembles the “skin” of a hollow ball, and a smaller, more densely populated disc-shaped region known as the Hills cloud.
Despite the fact that no direct, unambiguous observations of the Oort cloud have been made to date, the consensus among investigators seems to be that the material in the cloud formed much closer to the Sun as part of the proto-planetary disc, but that the tidal effects of the migration of the planets’ orbits somehow ejected all of the material to its present position. Nevertheless, the Oort cloud is widely regarded as the source of all long-period comets, centaurs (planetoids), and Jovian-family comets that enter the solar system proper.
Because the outer Oort cloud is not strongly bound to the solar system, it is thought that the combined tidal effects of the Milky Way, passing stars, and internal collisions occasionally perturb individual objects in the Hills cloud to the point where they are captured by the combined gravitational pull of the solar system, where they then become short- period comets. However, based on investigations of individual cometary orbits, some long period comets clearly originate in the shell-like outer Oort cloud.
Hypothesis on the Origin of the Oort Cloud
In an attempt to explain the origin of comets, the Estonian astronomer Ernst Öpik postulated in 1932 that they must originate from an area, or swarm, right at the edge of the solar system, and further, that the cloud or reservoir of cometary nuclei must be rotating with the solar system. At about the same time, the Dutch astronomer Jan Oort arrived at much the same conclusion independently, although Oort was attempting to solve a paradox rather than discover the origin of comets.
The paradox Oort was concerned with involved the fact that as comets approach the Sun, they must lose mass because of the continual sublimation of material on the one hand, and that since their orbits are unstable, they must be either destroyed by the Sun, broken up by collisions with planets, or ejected from the solar system by the combined tidal effects of planetary perturbations, on the other. In practice then, Oort reasoned that no cometary nucleus could thus have formed while in its present orbit, but must instead have been held captive in a remote reservoir, or swarm of similar bodies for much, if not most of its existence after its formation.
Of the two main cometary classes, those with orbits that follow the ecliptic, and those that have long-term orbits of several thousand AU’s that originate from all points in the sky, Oort noticed a peak in the numbers of long-period comets that have their aphelia at about 20,000 AU, a circumstance that pointed to the presence of a vast swarm of objects at that distance. Moreover, the random origin of long-period comets suggested that the swarm of objects were distributed isotropically, or spherically.
Structure and Composition of the Oort Cloud
While estimates on the dimensions of the Oort cloud varies widely, most investigators place the inner edge of the cloud at between 2,000- and 5,000 AU, with the outer limit at anywhere between 100,000 and 200,000 AU away from the Sun [Harold F. Levison; Luke Donnes (2007).
In general terms, the structure can be split into two distinctly different areas- the spherical Oort cloud proper that stretches between roughly 20,000 and 50,000 AU, and a more compacted, torus-shaped inner area from about 2,000 to 20,000 AU, from where the Oort cloud proper starts, and which is only tenuously bound to the Sun. This area is thought to be the origin of all long-period comets that penetrate the inner solar system.
The inner, more compact cloud, named after Jack G. Hills who first proposed the existence of an inner structure, is thought to be inhabited by several trillion objects that are continually being ejected into the outer Oort cloud, and is considered to be the main source of material for the outer cloud, which explains the continued existence of the outer cloud even so long after the formation of the solar system.
Although the collective mass of all the objects in the outer Oort cloud is not known with any degree of certainty, it is thought that the region is inhabited by several trillion objects, of which several billion are larger than approximately 20 km (12 miles) in diameter, and that the distance between individual objects must be tens of millions of kms to account for the large area the structure occupies.
Nonetheless, calculations that assume Halley’s Comet to be representative of the Oort cloud population a whole, put the combined mass of all the material in the cloud at about five Earth masses, or 3×1025 kg (6.6×1025 lbs) [Paul R. Weissman (1983). “The mass of the Oort Cloud”. Astronomy and Astrophysics 118 (1): 90–94.]. Previous estimates of the mass of the Oort cloud assumed it to be about 380 Earth masses; however, revised, and improved knowledge about the size, mass, and distribution of long-period comets have forced the mass estimated downward. The mass of the inner, Hills cloud remains unknown.
Origin of the Oort Cloud
The Oort cloud is assumed to be the remains of the proto-planetary disc from which the planets formed about 4.6 billion years ago, but that the bulk of the present Oort cloud formed much closer to the Sun. The most widely held view is that the young gas giants Jupiter, Saturn, Uranus, and Neptune collectively ejected the much less massive material out of the infant solar system into very long parabolic orbits.
However, a competing hypothesis that is based on recent research by NASA [“The Sun Steals Comets from Other Stars”. NASA. 2010], holds that much of the bulk of the objects in the Oort cloud originates from an interaction, or exchange of material between the proto-planetary discs of stars that formed in a cluster of about 400 stars of which the Sun was a member, and that the Sun managed to “hang” onto much of the material when the cluster disintegrated. This view suggests that as a result of the cluster’s disintegration, much, if not the major part of the objects in the Oort cloud could be shown not to have originated around the Sun.
On the strength of intricate, and enhanced computer models, Harold F. Levinson et al showed in 2010 that while the Sun was in its “birth cluster” it captured as much as 90% of the objects in the current Oort cloud from the proto-planetary discs of sibling stars. Enhanced computer simulations of the history and evolution of the solar system also suggest that the Oort cloud reached its most massive state about 800 million after its formation, after which depletion started to overtake the rate of supply.
Further modelling and simulations performed by Julio Ángel Fernández also suggested that the Hills cloud, or disc shaped formation within the larger Oort cloud, is the primary source of supply of objects to the Oort cloud proper. Based on results obtained from his simulations, Fernández showed that approximately 50% of the objects in the inner Hills cloud are eventually ejected into the Oort cloud proper, that about 25% are pulled inward towards the orbit of Jupiter, and that the remaining 25% are ejected out of the cloud on hugely hyperbolic orbits. Fernández also showed that in another 2.5 billion years, as much as 33% of the objects in the Hills, or scattered cloud will most likely end up in the Oort cloud proper.
Enhanced computer modelling also showed that the high rate of collisions between objects during the formation of the solar system destroyed more cometary nuclei before they could reach the relative stability of the Oort cloud than was previously thought. In practice, this means that the projected mass of the Oort cloud is substantially less than the estimated 50 to 100 Earth-masses of material that is thought to have been ejected during the formation of the solar system.
The spherical shape of the Oort cloud is thought to be the result of interactions between the tidal effects of the Milky Way Galaxy and those of passing stars that modified the hyperbolic orbits of objects in the cloud into more circular orbits. This view is based on the fact that since the Hills cloud is much more strongly bound to the solar system than the Oort cloud proper, the effects of passing stars in the original cluster that shaped the Oort cloud did not deform the Hills cloud in the same way, which is why it still retains its disc-like structure.
Future Exploration of the Oort Cloud
To date, no space craft have even reached the immediate vicinity of the Oort cloud. Even Voyager 1, the spacecraft with the highest solar recessionary speed will take another 300 years before it reaches the inner regions of the cloud. Assuming also that it will maintain its current speed of 38,610 miles per hour (62,136 km/h), which equates to 11 m/sec, or 17 km/sec, it will take Voyager roughly 30,000 years to cross the outer Oort cloud.
In practice, this means that eye-ball exploration of the Oort cloud is at present beyond our means; however, one possible way around the time/distance problem is the construction of a craft that is equipped to carry a sail that will use the solar wind for propulsion, much like a wind-powered yacht. Current estimates put the traveling time of such a craft to the Oort cloud at around 30 years.