The Kuiper BeltEarly expectationsThe edge of the solar system?The structure of the Kuiper beltThe PlutinosThe scattered beltThe classical Kuiper beltPushing it all outThe current storyThe Kuiper Belt On January 10th, 1992, a freighter en route from Hong Kong to Tacoma, Washington got caught in a storm in the northern Pacific ocean and lost overboard shipping containers filled with 29,000 plastic bathtub toys. Ten months later these brightly colored ducks, turtles, beavers, and frogs began showing up on beaches up and down the coast of Alaska. Curtis Ebbesmeyer and James Ingraham, Seattle oceanographers who had been studying wind and sea circulation in the Northwest Pacific by systematically dropping small numbers of labeled floats from fixed locations and hoping for their recovery, quickly realized that they had a massive inadvertent experiment on their hands. While large computer models can predict the physics behind the winds and currents, the large numbers of different interactions that occur make precise prediction difficult. The bathtub toys provided a windfall of data that would allow them to chart out winds and currents and understand the complex interactions in more detail than was previously possible with their more limited data sets. Like oceanographers with their limited numbers of data point to understand the complexities of currents, astronomers studying the formation and evolution of the outer solar system have for many years had few data points around which to spin their theories. There are 4 giant planets, each created and moved around by a complex suite of interactions. Trying to piece together all of these different interactions to discern the history was like trying to use just four large shipwrecks washed up on shore to understand the flows of all of the oceans’ currents. The breakthrough for astronomers came a few months before the first plastic ducks hit the beaches near Sitka. David Jewitt of the University of Hawaii and Jane Luu then of the University of California at Berkeley, after many nights of searching, found a single faint slowly moving object in the sky. After following it for a few days it became clear that they had found the first object beyond Neptune since Clyde Tombaugh had discovered Pluto in 1929. Today more than 800 additional members of what is now known as the Kuiper belt have been discovered in the outer solar system, and their existence is changing the picture of the dynamical evolution of the outer solar system that held just a decade ago. These objects behave in many ways like test particles which trace the gravitational effects of the rearrangements and perturbations of the giant planets. The existence of hundreds of such objects strewn throughout the outer solar system gives the concrete data which allows us to attempt to trace the history of the outer solar system as surely as tracing the routes of hundreds of plastic bathtub toys washed on beaches shows the winds and currents of the northern Pacific. Early expectations The prediction of the existence of a belt of small bodies beyond the orbit of Neptune was made in 1950 by Gerard Kuiper using a seemingly weak but ultimately correct line of argument. Kuiper proposed a method to conceptually reconstruct the initial disk of gas and dust from which the entire solar system formed. He began by taking Jupiter, smashing it flat, and spreading that entire mass out into an annulus between the orbits of Jupiter and Saturn. This annulus now represents the region of the nebula that went into making Jupiter. We know, however, that some material that was in initially in this region of the nebula must have been lost, since Jupiter has a higher abundance of elements heavier than hydrogen and helium than does the sun. So we add a little more mass to the annulus to make up for these lost elements and bring that region of the theoretical nebula to solarcomposition. Now we do the same to all of the giant planets and then to the terrestrial planets (the terrestrial planets have lost almost all of their hydrogen and helium, so a large amount of extra material has to be added in). We now have an approximate reconstruction of the mass distribution of the initial nebula (Figure 1). Figure 1 An estimate of the surface density of the nebular disk from which the planets formed. Distance from the sun is in astronomical units (AU), where 1 AU is the distance from the earth to the sun. The density estimate is made by spreading the total mass in each planet into an annulus stretching to the next nearest planets and adding a little extra to account for what was thought to have been lost. In 1950 Gerard Kuiper argued that the solar system shouldn’t abruptly end beyond Neptune and proposed the existence of a belt of small unseen bodies which we now know as the Kuiper belt. Kuiper noted that the surface density of the nebula smoothly dropped from the inside to the outside until, beyond Neptune, the density plummeted. He reasoned that the nebula should not have an abrupt edge and that beyond Neptune was a realm where densities were never high enough to form large bodies, but where small icy objects exist instead. Furthermore, he suggested, thisregion could be the source for the comets which periodically come blazing through the inner solar system. Almost 4 decades later, Martin Duncan of Queen’s College and colleagues, taking advantage of the growing power of computers for dynamical simulations of the long-term gravitational influence of planets, showed that one class of comets—the Jupiter-family comets—were best explained by the existence of a band of small bodies just beyond the orbit of Neptune, which they named the Kuiper belt. Jewitt and Luu found the first object in this hypothesized Kuiper belt just 6 years later. The edge of the solar system? One of the first surprises after the discovery of the Kuiper belt was that it appears to contain only about 1% of the mass needed to make up for the deficit noted by Kuiper. Even more interesting, early studies by Alan Stern at the Southwest Research Institute suggested that the Kuiper belt did not even contain enough mass to have formed itself. Stern pointed out that for the largest Kuiper belt objects build up from the gradual accumulation of the smaller objects—the typical way in which solid bodies in the solar system are thought to form— would have taken longer than the age of the solar system. An apparent resolution to this discrepancy
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