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Case Study: Gerard Kuiper and the Trans-Neptunian Comet Belt

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Oblique view of the solar system, showing the main concentration of Kuiper Belt objects beyond the orbit of Neptune. The outer edge of the Kuiper Belt is not known. The orbit of Neptune is 60 Astronomical Units in diameter. Photo © American Museum of Natural History.


The census of the solar system is far from complete. When Pluto was found in 1930, it was hailed as the most distant planet. However, it turned out to be smaller than the Earth’s Moon, and there remained a puzzling lack of sizeable objects beyond the orbit of Neptune. The comets, however, had long brought us hints of what lay in the farthest reaches of the solar system.

Comets are dusty snowballs on elliptical orbits around the Sun. When a comet enters the inner solar system, sunlight evaporates the surface ice and blows back a prominent tail of gas and dust. If we list comets according to their orbital periods, two distinct groups emerge. Short-period comets orbit the Sun in the same direction as the planets, and have periods less than about 200 years. In contrast, long-period comets have extremely stretched-out elliptical orbits, with periods often more than a million years, and they enter the inner solar system randomly from all directions. Any long-period comet will be seen by us only once. Indeed the orbits of these comets must take them hundreds to thousands of times farther from the Sun than the orbit of Pluto. Where do they come from?

In 1950, the Dutch astronomer J. H. Oort proposed that all comets come from an enormous spherical cloud of objects extending almost halfway to the nearest stars and only weakly bound by the Sun’s gravity. This cloud, now called the Oort cloud, is occasionally perturbed gravitationally as it moves through the Galaxy. Such disturbances hurl comets from the cloud into the inner solar system, where we can see them.

Most astronomers immediately accepted the existence of the Oort cloud, but its origin remained unclear. Furthermore, while the theory accounted for the long-period comets, it did not adequately explain how short-period comets acquired their relatively small orbits with a preference for the plane of the solar system. It was assumed that short-period comets must be long-period comets that had passed close enough to Jupiter for its gravity to pull them into the plane of the solar system and shorten their orbits. But the idea was never verified quantitatively.

In 1951, the Dutch-American astronomer Gerard Kuiper wrote an influential paper on the origin of the solar system. Like others, he assumed that the planets had condensed from the solar nebula, a disk-shaped cloud of gas and dust in orbit around the young Sun (an idea first proposed by the philosopher Immanuel Kant in 1755). When Kuiper estimated the original amounts of material in the solar nebula at various distances from the Sun, he found that the distribution increased to a peak at the distance of the giant planets Jupiter and Saturn and then declined beyond the orbit of Neptune. His model of the solar nebula beyond Neptune had insufficient mass to form another large planet, but a smooth distribution of leftover gas and dust extended to greater distances. Kuiper calculated that this material would have condensed to form billions of small icy bodies with the size and composition of comets. He also concluded that gravitational perturbations by the planets would have thrown most of these bodies out to the distance of the Oort cloud, “where stellar perturbations would have altered their orbits once more, making them rounder and of random inclination.”

But not all the comets would have been ejected to the Oort cloud. Kuiper assumed that beyond the orbit of Pluto, “remnants of the circular comet ring are still present.” This hypothetical donut-shaped region of comets beyond Neptune became known as the Kuiper Belt. (It was also called the Edgeworth-Kuiper Belt, in recognition of the Irish astronomer Kenneth Edgeworth, who had earlier made a similar suggestion.)

In the mid-1980s, David Jewitt and Jane Luu began to search for the small bodies that might populate a distant Kuiper Belt. They expected that modern observing techniques would be able to render such faint objects visible. They took hundreds of pictures for many years, scouring them for any signs of faint, slowly moving objects. Finally, in late 1992, after a night of observation on the peak of Mauna Kea in Hawaii, they compared two images taken fifteen minutes apart and saw a faint object that had moved slightly through the sky. Jewitt says, “We both fell silent.” After taking several more images of the same part of the sky, there could be no doubt: they had discovered the first Kuiper Belt Object (KBO). It was a small world of ice and dust, perhaps 250 kilometers across, with an orbit somewhat larger than Pluto’s. To date over 200 KBOs have been found. Only the largest ones can be observed, but estimates suggest that the KBOs number in the billions.

The Kuiper Belt is the oldest surviving remnant of the original solar nebula. It represents the original source of the far more distant Oort cloud comets, as Kuiper had proposed. Furthermore, the Belt may also be the source of some of the short-period comets. Most astronomers (including Kuiper) had thought that these were long-period Oort cloud comets perturbed into short-period ones by Jupiter, but calculations have shown that this mechanism is inefficient. Instead, it was found that the gravity of Neptune can hurl enough KBOs into the inner solar system to account for the population of short-period comets. Since KBOs already orbit in the same direction as the planets, this mechanism explains the direction of short-period comets moving around the Sun. And since most of the known KBOs already have periods of only a few hundred years (unlike the long-period comets), perturbation by Neptune can convert enough of them to account for the observed short-period comets.

The discovery of the Kuiper Belt has made profound changes in our picture of the outer solar system. Even Pluto is now regarded by many astronomers as a giant comet—the largest of the KBOs. As observations continue to bring more of these distant objects into view, we should count on new surprises.

This is an excerpt from COSMIC HORIZONS: ASTRONOMY AT THE CUTTING EDGE, edited by Steven Soter and Neil deGrasse Tyson, a publication of the New Press. © 2000 American Museum of Natural History. 

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