Jan Hendrik Oort propounded the theory that comets visible from Earth originate in a massive cloud of comets one light-year from the Sun. The theory both explains the observable features and behavior of comets and provides a better picture of the origins of the solar system.
Comets have fascinated humans for millennia. For most of that time, the appearance of a comet was considered a portent of disaster. When the great comet of 1066 made its appearance, William the Conqueror, claimant to the throne of England, shrewdly informed his troops that the comet signified defeat of the English defenders; it also, he said, implied that he would become king of England. In the 1500’s, comets became the subject of scientific inquiry. Edmond Halley
As technology improved, additional information was gathered about the shapes of comets’ orbits, the directions in which they orbit the Sun, why they have two tails, and why the tails always point away from the Sun. While some comets are repeat visitors to the inner solar system, others, such as Kohoutek
Fred Whipple first described comets as dirty snowballs in 1949. They are usually composed of ices of ammonia, methane, and other compounds. Mixed in with these ices are pieces of rocky material. Each time a comet comes close to the Sun, the heat from the Sun melts some of the icy material. This forms a coma, or atmosphere, around the comet body, or nucleus, that may be kilometers to tens of kilometers in diameter. The solar wind, which is composed of gases expelled by the Sun at high speeds, pushes the gases forming the coma away from the nucleus to form one of the comet’s tails. Light pressure forms the other tail from the coma’s dusty components. This explains why the tails always point away from the Sun, since it is the source of the sunlight and solar wind that form the tails.
With the melting and vaporizing of its ices, the comet loses some of its mass each time it makes a pass by the Sun. Even if it lost only one one-millionth of its mass per orbit, Halley’s comet would disappear in 76 million years. There are objects that orbit the Sun that may be burned-out comets. This is possible, since once the ices melt, most of the rocky material remains. The actual origins of these objects remain unknown.
Comets may be classified by the length of their orbital periods. Long-period comets have a period greater than two hundred years, and short-period comets less than two hundred years. Short-period comets were once long-period comets, but the gravitational attraction of the outer planets, such as Jupiter, has altered their orbits. Long-period comets have greatly elongated elliptical orbits that take them far beyond the orbit of Pluto and the other dwarf planets. Even the orbit of Halley’s comet, with a period of only seventy-six years, reaches beyond the orbit of Neptune. This tendency of comets to have very elongated orbits is unique among bodies within the solar system; most other objects—the classical and dwarf planets, their satellites, the asteroids—have less elliptical orbits.
Another oddity of cometary orbits is that they can orbit the Sun in any direction. All planets orbit the Sun in a counterclockwise direction, as viewed from the direction of Earth’s North Pole. The planes of these planetary orbits are close to the plane of Earth’s orbit. In other words, they orbit the Sun in a limited range of directions. Comets can orbit the Sun at any angle. The orbital plane of Halley’s comet, for example, is inclined to Earth’s orbit by 18 degrees, and the comet orbits the Sun in the clockwise direction.
Jan Hendrik Oort first postulated the source of comets in 1950. He reasoned that comets came from a cloud of comets located a light-year from the Sun. The cloud should be a spherical shell billions of kilometers thick. The number of comets in this reservoir could be in the trillions. Although that seems to be a very large number, it represents a total mass similar to that of the Sun. The individual orbits of comets within this cloud should be elliptical, but not as elongated as comets that closely approach the Sun. The orbital period for a comet at a distance of a light-year is 15 million years. Its temperature should be close to absolute zero, since it would receive very little energy from the Sun.
To verify this theory, the known facts about comets can be checked. A comet in Oort’s cloud would move around the Sun at a velocity of several meters per second, compared to tens of kilometers per second for an object like Earth. If a comet comes close to another comet, they will gravitationally interact and will change their orbital direction and speed. If a comet slows down, it will move closer to the Sun, and its orbit will become elongated. More gravitational encounters with the outer planets would change its orbit even more, causing it to move closer to the Sun. The point of its orbit farthest from the Sun, its aphelion, would also decrease, until it would be recognized as a long-period comet. Additional encounters with Jupiter would alter the comet’s orbit to that of a short-period comet.
Since this theoretical example comet is coming toward the Sun from such a large distance, it can orbit the Sun at any angle or direction, which explains comets’ diverse orbital behavior in general. They should be composed of icy and rocky material, since that is the average composition of the material in the solar system. The ice is caused by the low temperature of the outer reaches of the solar system. In the inner solar system, comets burn out over time, but they are replaced by new comets from the Oort cloud. Only several per year would need to be dislodged from the cloud for the number of inner-solar-system comets to remain relatively constant.
The idea of Oort’s cloud being the source of the solar system’s comets fits in well with the currently accepted model for the origin of the solar system. It is believed that the solar system arose from a slowly rotating cloud of gas and dust several light-years across and several times the mass of the present solar system. The cloud started to collapse under the gravitational attraction of its various particles. A supernova explosion near the cloud may have sent a shock wave through the cloud that helped start the process. As the cloud collapsed, it spun faster like a skater spins faster and pulls in her arms. As it spun, the collisions of the particles caused the cloud to become more disk-shaped.
Some of the collisions resulted in the particles sticking together and thereby growing larger. The inner portion of the cloud grew hotter, since the collisions occurred more often there. Only rocky and metallic material could form under those high-temperature conditions, while icy compounds condensed out in the outer portion of the solar system. As the particles grew, they reached a size of several kilometers in diameter, a stage referred to as planetesimals. Some of these planetesimals would collide and grow in size to become the satellites and planets. Others would interact gravitationally and would be thrown away from the Sun. They could escape totally or become part of Oort’s cloud.
This nebular theory
Another implication of Oort’s cloud is its possible involvement in the periodic mass extinctions that have occurred on Earth. Sixty-five million years ago, the dinosaurs and many other creatures became extinct. The impact of a ten-kilometer diameter asteroid or comet may have been the cause. Other mass extinctions have occurred at roughly 26-million-year intervals. An unknown planet or other body with a 26-million-year period of revolution may pass through the Oort cloud and cause many comets to start toward the inner solar system. There, some could collide with Earth and cause the mass extinctions by drastically changing the climate of Earth. The idea of Oort’s cloud explains many of the characteristics of comets and agrees well with the nebular theory of the solar system’s origin.
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Brandt, John C., and Robert D. Chapman. Introduction to Comets. 2d ed. New York: Cambridge University Press, 2004. Exhaustive introduction to the study of comets. Among the topics discussed are the components and compositions of comets, their paths, and their origins. Bibliographic references and index. -
Delsemme, A. “Comets and the Origin of the Solar System.” In The Origin of the Solar System, edited by S. F. Dermott. New York: John Wiley & Sons, 1978. This is a short article dealing with the origin of comets, their chemical composition, dust and gas amounts, and their thermal history. It is somewhat technical, but a reader with some science background should comprehend it. It also has many references, including classic papers by Fred Whipple. -
Fernandez, Julio Angel. Comets: Nature, Dynamics, Origin, and Their Cosmogonical Relevance. Dordrecht, the Netherlands, 2005. Covering both the specific and the general, this text explains what comets are and how they work, as well as what they mean to larger issues in astronomy and cosmology. Bibliographic references and index. -
Raup, David M. The Nemesis Affair. New York: W. W. Norton, 1986. This book is the story of the investigations into how the mass extinctions on Earth were caused by comets from Oort’s cloud hitting Earth. It is a good discussion of how science and scientists really work. -
Van Flandern, Tom. “Where Do Comets Come From?” Mercury 11 (November/December, 1982): 189-193. Science is not static. New theories are always being developed, and this article proves that point since it proposes a new origin for comets in the solar system. The author contends that comets originated when a “planet,” in or near the asteroid belt in the past, fragmented. -
Whipple, Fred. “Discovering the Nature of Comets.” Mercury 15 (January/February, 1986): 2-9. This article is an adaptation of Whipple’s book The Mystery of Comets (1985) and provides an introduction to comets. He also presents his personal thoughts on how he developed his model of comets. -
_______. “The Nature of Comets.” Scientific American 230 (February, 1974): 48-57. Since Whipple is the developer of the dirty-snowball description of comets, it is fitting that he wrote this article about comet Kohoutek and the nature of comets. Although Kohoutek was a great disappointment, it furnished scientists with a wealth of information about the primordial solar system and the origin of comets in the Oort cloud.
Von Weizsäcker Forms His Quantitative Theory of Planetary Formation
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Reber Publishes the First Radio Maps of the Galaxy
Ryle’s Radio Telescope Locates the First Known Radio Galaxy
Hale Constructs the 200-Inch Telescope
De Vaucouleurs Identifies the Local Supercluster of Galaxies
Franklin and Burke Discover Radio Emissions from Jupiter
Mössbauer Effect Is Used to Detect Gravitational Redshifting
Bell Discovers Pulsars
Wheeler Refers to Collapsed Stars as “Black Holes”