The Oort cloud is a vast swarm of some 2 trillion comets orbiting our star in the most distant reaches of our solar system, extending from beyond the orbits of Neptune and Pluto out to 100,000 times the Earth-Sun distance—nearly one-third the distance to the nearest star. While the planets are confined to a flattened disk in the solar system, the Oort cloud forms a spherical shell centered on the Sun, which gradually flattens down to an extended disk in the inner region, called the Kuiper belt. Bright comets observed through telescopes or with the unaided eye get perturbed out of the Oort cloud or Kuiper belt, and become visible when they get close to enough so that the Sun's energy can transform the surface ices into gases. These gases drag off the embedded dust, and we see the light reflected from the dust as a tail.
Comets are the leftover icy building blocks from the time of planet formation, which formed in the region of the outer planets. Essentially these comets are dirty snowballs, composed primarily of water ice, with some carbon monoxide and other ices, in addition to interstellar dust. When their orbits passed close enough to the giant planets to be affected, some were thrown toward the Sun and some were tossed outward toward the distant reaches of the solar system, the spherical swarm we now call the Oort cloud. Some of the comets sent inward hit the inner rocky planets, and probably contributed a significant amount of ocean water and organic material, the building blocks of life, to Earth. Comets that live in the Oort cloud are especially important scientifically because they have been kept in a perpetual deep freeze since the formation of our solar system 4.6 billion years ago. This means that they preserve, nearly intact, a record of the chemical conditions during the first few million years of the solar system's history, and can be used to unravel our solar system's origins much like an archaeologist uses artifacts to decipher an ancient civilization.
The Oort cloud was "discovered" by Dutch astronomer Jan Hendrik Oort in 1950, not through telescopic observations, but through a theoretical study of the orbits of long-period comets (comets with periods greater than 200 years). Long-period comets can have orbits ranging from eccentric ellipses to parabolas to even modest hyperbolas. While trying to explain the distribution of these orbits (which were mostly nearly parabolic or hyperbolic), Oort concluded that the only explanation was that the source of these comets had to be a massive cloud of comets surrounding the solar system. These comets would be fed into the region of the planets as the motion of the solar system through the galaxy caused the solar system to pass relatively close to stars. The slight change in the gravitational acceleration from these stars was enough to send some distant comets into orbits that brought them into the inner solar system.
Oort's remarkable discovery was made with only a few handfuls of comet observations. Since then, precise observations of comet orbits and new modern computer models have shown not only that his ideas were correct but also that the Oort cloud can be divided into different regions: the outer Oort cloud, acted upon by passing stars; the inner Oort cloud, which is close enough to the Sun (perhaps 2,000 to 15,000 Earth-Sun distances) that the comets are not affected by gravitational interactions, and finally the flattened innermost region—the Kuiper belt. Kuiper belt comet orbits can be perturbed through interactions with the outer planets, and these comets then become observable as short-period comets. Because the Kuiper belt is much closer to the Sun, the world's largest telescopes began directly observing these comets in the late-twentieth century. The first Kuiper belt object was discovered in 1993, and by 2002 more than 500 such objects were discovered.
see also Comets (volume 2); Kuiper Belt (volume 2); Orbits (volume 2); Planetesimals (volume 2); Small Bodies (volume 2).
Karen J. Meech
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Oort, Jan H. "The Structure of the Cloud of Comets Surrounding the Solar System and a Hypothesis Concerning Its Origin." Bulletin of the Astronomical Institute of the Netherlands 11 (1950):91-110.
Weissman, P. R. "The Oort Cloud and the Galaxy: Dynamical Interactions." In The
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