Mankind’s Theory: Comets and the Oort Cloud

Mankind’s Theory: Comets and the Oort Cloud

“Comet, an astronomical body of small mass, moving about the sun in a more or less elongated orbit. When it is sufficiently close to the sun, icy materials in the main mass, or nucleus, are vaporized by sunlight and form a hazy envelope of gases and finely divided dust particles. The presence of this diffuse envelope, which tends to obscure the nucleus, is a characteristic and defying feature. The dust and gases may then form one or more tails, which are always directed more or less away from the sun. The term comet derives from a Greek word meaning “long-haired,” which is descriptive of the following appearance of the tail of a bright comet.

Comets were formerly of interest to astronomers principally because of problems connected with their motions. They are now recognized as representing probably the least-changed available sample of the primordial material from which the sun and planets were formed. Thus they are studied today in order to understand their physical and chemical characteristics.

Properties

Orbits. Comets are generally classified, according to the size and shape of their orbits, into short period and long period objects. The former move in elliptical orbits that require less than 200 years per revolution. Some of them have been observed repeatedly for many revolutions as they pass the earth and sun. The long-period comets move in much larger and more elongated orbits that are nearly parabolic in shape. Such comets come to the vicinity of the sun only at intervals of thousands or millions of years. A recent catalog of orbits listed 135 short-period comets and 613 that moved in nearly parabolic orbits.

The great majority of short-period comets have periods between 3 and 9 years long. Of these, over 120 comets known as the Jupiter family have aphelia that are located close to the orbit of Jupiter. (Aphelion is the orbital point most distant from the sun.) Essentially all the remaining comets of short period appear to have passed close to Jupiter at one time or another. Although associations of similar comet families with other planets have been suggested, their reality is doubtful. The probability of the dominance of Jupiter in shaping the smaller orbits seems overwhelming.

The orbit planes of short-period comets also show a concentration toward the average plane of motion of the major planets. Practically all these comets revolve about the sun in the same direction, as do the planets. Comet Halley, which revolves in the retrograde orbit inclined some 18 degrees to the plane of the earth’s orbit, is one noteworthy exception.

The orbits of the long-period comets, on the other hand, reach far beyond the domain of the major planets and out toward interstellar space. Such orbits are distributed essentially at random with respect to the inclination of the orbit planes to the plane of the earth’s orbit around the sun. The apparent distribution of orientations of the long axes of the comets orbits in their planes is more complex, being affected by such factors as the motion of the sun in the Milky Way Galaxy and effects of observing conditions on comet discoveries.

Storage Clouds of Comets. From a study of the sizes and shapes of the best-determined nearly parabolic orbits, freed from the disturbing effects of the major planets, the Dutch astronomer J. Oort concluded in 1950 that fresh comets must come from a vast storage region, later called the Oort cloud, at the far outer edges of the solar system. He supposed that large numbers of comets move slowly in a sort of deep freeze in this region, continually stirred by gravitational disturbances produced by passing stars. Occasionally a comet would be so deflected as to be sent into the inner solar system, where it could be observed. To account for the frequency with which such comets were seen, Oort calculated that there must be some 100 billion comets in this distant, nearly spherical, cloud. Later calculations showed that galactic tides and giant interstellar clouds, as well as passing stars, disturb the motion of the distant comets. Their number must even be larger than Oort supposed, and their total mass is likely to be at least several times the mass of the earth.

Many comets would have been lost from distant cloud over time to interstellar space. Further, there are difficulties in capturing comets from nearly parabolic obits into small elliptic ones efficiently enough to match the observed numbers of short-period comets. These, and some other considerations, led to the idea of an inner storage cloud. Such a cloud is thought to extend from just beyond the planets to merge into the distant Oort cloud. The inner portion probably is substantially flattened to the average plane of the planetary system. Almost certainly the inner storage cloud contains a mass of comets substantially larger than that of the distant Oort cloud. On relatively rare occasions the inner cloud might be disturbed by the passage of a star unusually close to the sun. As a consequence, showers of comets could be sent into the inner solar system, some of them to be captured by Jupiter and the large planets into short period orbits. Others would be thrown outward to repopulate the Oort cloud.

As comets repeatedly penetrate the part of solar system closest to the sun, their orbits are gradually changed by the gravitational attraction of the planets, especially Jupiter. Some are ejected from the solar system into interstellar space. Some return to the Oort cloud. And some, especially those that move in planes close to those of the principal planets, may be deflected into small, distinctly elliptical orbits like those of the short-period comets.

Formation of Comets. Comets probably formed in the outer portions of the solar nebula, either in the vicinity of the outermost planets or perhaps as far from the sun as the proposed massive inner cloud of comets. Some old grains that condensed in dense cold parts of interstellar clouds may have survived the formation of he solar nebula to be incorporated virtually unchanged into the nuclei of comets.

The early history of the solar system is very hard to decipher from the properties of the highly metamorphosed planets and asteroids that have long stayed close to the sun. If comets were formed at the same time as the planets but in the cold outer portions of the solar nebula, it seems very probable that some of the record of those early times is locked in the present chemical and physical structure of the comets. Thus they have become targets for intensive scientific investigation from earth and from spacecraft.”

Elizabeth Roemer

University of Arizona

Bibliography

Brandt, John C., and Chapman, Robert D., Introduction to Comets (Cambridge 1982)

Hillebrandt, W., and Meyer-Hofmeister, E., eds., Physical Processes in Comets, Star and Active Galaxies (Springer-Verlag 1987)

Marsden, Brian G., Catalogue of Cometary Orbits, 5th ed. (Enslow 1986)

Whipple, Fred L., The Mystery of Comets (Smithsonian Institution Press 1985).

“Comets also are comic debris, probably planetesimals that originally resided in the vicinity of the orbits of Uranus and Neptune rather than in the warmer regions of the asteroid belt. Thus, the nuclei of the comets are icy balls of frozen water, methane, and ammonia, mixed with small pieces of rock and dust, rather than the largely volatile-free stones and irons that typify asteroids. In the most popular theory, ice planetesimals in the primitive solar nebula that wandered close to Uranus or Neptune but not close enough to be captured by them were flung to great distances from the Sun, some to be lost from the solar system while others populated what was to become a great cloud of cometary bodies, perhaps 10 trillion in number. Such a cloud was first hypothesized by the Dutch astronomer Jan Hendrik Oort.

In the original version of the theory, the Oort cloud extends tens of thousands of times farther from the Sun than the Earth, a significant fraction of the way to the nearest stars. Random encounters with passing stars would periodically throw some of the comets into new orbits, plunging them back toward the heart of the solar system. As a comet nears the Sun, the ice begins to evaporate, loosening the trapped dust and forming a large coma that completely surrounds the small nucleus, which is the ultimate source of all material. The solar wind blows back the evaporating gas into an ion tail, and radiation pressure pushes back the small particle solids into a dust tail. Each solid particle is an independently orbiting satellite of the Sun, and the accumulation of countless such passages by many comets contributes to the total quantity of dust particles and micrometoroids found in interplanetary space.

The total mass contained in all the comets is highly uncertain. Modern estimates range from 1 to 100 Earth masses. Part of the uncertainty concerns the reality of a hypothesized massive “inner Oort Cloud” or “Kuiper belt” (if the distribution is flatten) of the comets that would exist at distances from the Sun 40 to 10,000 times that of the orbit of Earth. At such locations, the comets would not be much perturbed by the typical passing stars nor by the gravity of the planets of the solar system, and the comets cloud reside in the inner cloud or belt for long periods of time without detection. It has been speculated, however, that a rare close passage by another star (possibly an undetected companion of the Sun) may send a shower of such comets streaming toward the inner solar system. If enough large cometary nuclei in such showers happen to strike the Earth, the clouds of dust ash that they would raise might be sufficient to trigger mass biological extinctions. As event of this kind appears especially promising for explaining the relatively sudden disappearance from Earth.”

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