Oort cloud

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This article is about the outer Oort cloud. For the inner Oort cloud, see Hills cloud.
space probe Voyager 1 by 2012–2013, which will reach the Oort cloud in about 300 years.
An artist's impression of the Oort cloud and the Kuiper belt
(inset); the sizes of objects are over-scaled for visibility.

The Oort cloud (

inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.[5]

The cloud is thought to comprise two regions: a disc-shaped inner Oort cloud aligned with the solar ecliptic (also called its Hills cloud) and a spherical outer Oort cloud enclosing the entire Solar System. Both regions lie well beyond the heliosphere and are in interstellar space.[4][6] The innermost portion of the Oort cloud is more than a thousand times as distant from the Sun than the Kuiper belt, the scattered disc and the detached objects—three nearer reservoirs of trans-Neptunian objects.

The outer limit of the Oort cloud defines the

short-period comets appear to have come from the Oort disc. Other short-period comets may have originated from the far larger spherical cloud.[4][9]

Astronomers hypothesize that the material presently in the Oort cloud formed much closer to the Sun, in the

long-period and Halley-type comets, which are eventually consumed by their close approaches to the Sun after entering the inner Solar System. The cloud may also serve the same function for many of the centaurs and Jupiter-family comets.[9]

Types of distant minor planets

Development of theory

By the turn of the 20th century, it was understood that there were two main classes of comet: short-period comets (also called

Kuiper cliff around 50 AU from the Sun (the orbit of Neptune averages about 30 AU and 177P/Barnard has aphelion around 48 AU). Long-period comets, on the other hand, travel in very large orbits thousands of AU from the Sun and are isotropically distributed. This means long-period comets appear from every direction in the sky, both above and below the ecliptic plane.[11]
The origin of these comets was not well understood, and many long-period comets were initially assumed to be on parabolic trajectories, making them one-time visitors to the Sun from interstellar space.

In 1907, Armin Otto Leuschner suggested that many of the comets then thought to have parabolic orbits in fact moved along extremely large elliptical orbits that would return them to the inner Solar System after long intervals during which they were invisible to Earth-based astronomy.[12] In 1932, the Estonian astronomer Ernst Öpik proposed a reservoir of long-period comets in the form of an orbiting cloud at the outermost edge of the Solar System.[13] Dutch astronomer Jan Oort revived this basic idea in 1950 to resolve a paradox about the origin of comets. The following facts are not easily reconcilable with the highly elliptical orbits in which long-period comets are always found:

Oort reasoned that comets with orbits that closely approach the Sun cannot have been doing so since the condensation of the protoplanetary disc, more than 4.5 billion years ago. Hence long-period comets could not have formed in the current orbits in which they are always discovered and must have been held in an outer reservoir for nearly all of their existence.[14][15][11]

Oort also studied tables of

aphelia) cluster around 20,000 AU. This suggested a reservoir at that distance with a spherical, isotropic distribution. He also proposed that the relatively rare comets with orbits of about 10,000 AU probably went through one or more orbits into the inner Solar System and there had their orbits drawn inward by the gravity of the planets.[11]

Structure and composition

The presumed distance of the Oort cloud compared to the rest of the Solar System

The Oort cloud is thought to occupy a vast space somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)[11] from the Sun to as far out as 50,000 AU (0.79 ly) or even 100,000 to 200,000 AU (1.58 to 3.16 ly).[4][11] The region can be subdivided into a spherical outer Oort cloud with a radius of some 20,000–50,000 AU (0.32–0.79 ly) and a torus-shaped inner Oort cloud with a radius of 2,000–20,000 AU (0.03–0.32 ly).

The inner Oort cloud is sometimes known as the Hills cloud, named for Jack G. Hills, who proposed its existence in 1981.[16] Models predict the inner cloud to be the much denser of the two, having tens or hundreds of times as many cometary nuclei as the outer cloud.[16][17][18] The Hills cloud is thought to be necessary to explain the continued existence of the Oort cloud after billions of years.[19]

Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun. This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possibly Halley-type) comets inside the orbit of Neptune.[4] This process ought to have depleted the sparser, outer cloud and yet long-period comets with orbits well above or below the ecliptic continue to be observed. The Hills cloud is thought to be a secondary reservoir of cometary nuclei and the source of replenishment for the tenuous outer cloud as the latter's numbers are gradually depleted through losses to the inner Solar System.

The outer Oort cloud may have trillions of objects larger than 1 km (0.6 mi),[4] and billions with diameters of 20-kilometre (12 mi). This corresponds to an absolute magnitude of more than 11.[20] On this analysis, "neighboring" objects in the outer cloud are separated by a significant fraction of 1 AU, tens of millions of kilometres.[9][21] The outer cloud's total mass is not known, but assuming that Halley's Comet is a suitable proxy for the nuclei composing the outer Oort cloud, their combined mass would be roughly 3×1025 kilograms (6.6×1025 lb), or five Earth masses.[4][22] Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses,[23] but improved knowledge of the size distribution of long-period comets has led to lower estimates. No estimates of the mass of the inner Oort cloud have been published as of 2023.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.[24] However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid[25][26] in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids.[27] Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,[28] a conclusion also supported by studies of granular size in Oort-cloud comets[29] and by the recent impact study of Jupiter-family comet Tempel 1.[30]

Origin

The Oort cloud is thought to have developed after the

parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.[4][31]

Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart and it is suggested that many—possibly the majority—of Oort cloud objects did not form in close proximity to the Sun.[32] Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.[4]

Models by

periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.[33] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.[34]

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.[35] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.[4]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.[4] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.[36]

In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars."[37][38] In July 2020 Amir Siraj and Avi Loeb found that a captured origin for the Oort Cloud in the Sun's birth cluster could address the theoretical tension in explaining the observed ratio of outer Oort cloud to scattered disc objects, and in addition could increase the chances of a captured Planet Nine.[39][40][41]

Comets

centaurs.[43] These centaurs are then sent farther inward to become the short-period comets.[44]

There are two main varieties of short-period comet: Jupiter-family comets (those with

semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley's Comet, are unusual in that although they are short-period comets, it is hypothesized that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.[15] This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.[9]

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No dynamical process is known to explain the smaller number of observed comets than Oort estimated. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all

outer-planet region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994.[46] An example of a typical dynamically old comet with an origin in the Oort cloud could be C/2018 F4.[47]

Tidal effects

Main article: Galactic tide

Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the

outer Solar System. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational Galactic Center compresses it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun.[48] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 au, and marks the outer boundary of the Oort cloud.[11]

Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud by increasing the

perihelia (smallest distances to the Sun) of planetesimals with large aphelia (largest distances to the Sun).[49] The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.[50] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.[51]

Stellar perturbations and stellar companion hypotheses

Besides the

close proximity to other stellar systems. For example, it is hypothesized that 70 thousand years ago, perhaps Scholz's Star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect).[52] During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.[53] This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.[53][54]

In 1984, physicist Richard A. Muller postulated that the Sun has an as-yet undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt.[55][56] Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.[57] Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.[57]

A somewhat similar hypothesis was advanced by astronomer

all-sky survey using parallax measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis.[57] In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.[59]

Future exploration

Artist's impression of the Voyager spacecraft

Space probes have yet to reach the area of the Oort cloud. Voyager 1, the fastest[60] and farthest[61][62] of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years[6][63] and would take about 30,000 years to pass through it.[64][65] However, around 2025, the radioisotope thermoelectric generators on Voyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by Voyager 1. The other four probes currently escaping the Solar System have either already stopped functioning or are predicted to stop functioning before they reach the Oort cloud.

In the 1980s, there was a concept for a probe that could reach 1,000 AU in 50 years, called TAU; among its missions would be to look for the Oort cloud.[66]

In the 2014 Announcement of Opportunity for the

Kepler observatory could have been capable of detecting objects in the Oort cloud.[68]

See also

References

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Explanatory notes

  1. millennia as different stars pass the Sun
    and thus is subject to variation. Estimates of its distance range from 50,000 to 200,000 au.

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