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t TheKuiper Belt the picture that astronomers held just s a decade ago of the outer Solar Sys- t tem's dynamical evolution. In many )- ways, Kuiper belt objects behave like test particles that trace the gravita- e tional effects of the giant planets' re- '- MichaelE. Brown arrangements and perturbations. The v hundreds of such objects strewn I- throughout the outer Solar System it fin 10 January 1992, a freighter en route from Hong give cbncrete data that allow astronomers to trace the his- n \rf Kong to Tacoma,Washington, got caught in a storm in tory of that system-as surely as tracing the routes of hun- rt the northern Pacifrc Ocean, and shipping containers frlled dreds ofplastic bathtub toys washed on beachesshows the v wjth 29 000 plastic bathtub toys were lost overboard. Tbn winds and currents ofthe northern Pacifrc. months later, brightly colored ducks, turtles, beavers, and frogs began showing up on beaches all along the coast of Earlyexpectations Alaska. Seattle-basedoceanographers Curtis Ebbesmeyer The prediction of a belt of small bodies, or planetesimals, r- and James Ingraham quickly realized that they had a mas- beyond the orbit of Neptune was made in 1950 by Gerard p- sive inadvertent experiment on their hands. In their re- Kuiper, pictured in frgure 1, who used a seemingly weak a- search, the two had been studying wind and sea circula- but ultimately correct line of argument. Kuiper proposed IC tion in the northwest Pacific by'systematically dropping a method to conceptually reconstruct the initial disk of gas e- small numbers of labeled floats from fixed locations and and dust from which the entire Solar System formed. He d- hoping for their recovery.As they well knew, scientists un- began by taking Jupiter, smashing it flat, and spreading et derstood the physics behind the winds and currents, but its entire mass into an annulus centered around the orbit rct the large numbers of different interactions that occur ofthe giant planet. That annulus represented the region er made precise prediction difficult---even with the help of of the nebula that went into making Jupiter. Kuiper knew, rm large computer models. The bathtub toys provided a wind- however, that some material initially in that part of the if- fall of data that would allow the two oceanographers to nebula must have been lost, becauseJupiter has a higher at chart out winds and currents and understand the complex abundance ofelements heavier than hydrogen and helium fic interactions in more detail than was previously possible than the Sun does. So he added a little more mass to the te- with their more limited data. annulus to make up for the lost hydrogen and helium and m- Like oceanographers trying with a paucity of observa- bring that region ofthe theoretical nebula to solar compo- tions to understand the complexities of currents, as- sition. Kuiper applied the same procedure to the remain- )ss tronomers studying the formation and evolution of the ing giant planets and to the terrestrial planets. (The ter- in- outer Solar System have had, until recently, few data points restrial planets have lost almost all oftheir hydrogen and 55, around which to spin their theories. The outer Solar Sys- helium so a large amount of extra material had to be added 12) tem's four giant planets were each created and moved in.) His method yields an approximate reconstruction of ics around by a complex suite ofinteractions. Tlying to piece the mass distribution of the initial nebula similar to the re- together all ofthose interactions to discern a history would more modern reconstruction displayed in frgure 2. rics be like trying to use just four large shipwrecks washed Kuiper noted that the surfaee density of the nebula or- ashoreto understand the flows ofall ofthe oceans'currents. smoothly dropped from the inside to the outside until, be- tre The breakthrough for astronomers came a few months yond Pluto, the flensity plummeted. He reasoned that the ten before the frrst plastic ducks hit the beaches near Sitka, nebula should not have an abrupt edge and that beyond bto Alaska, in November 1992. After a long search, David Je- Pluto was a realm where densities were never high enough witt of the University of Hawaii and Jane Luu, then of the to form large bodies, but where small icy objects existed University of Califorriia, Berkeley, found a single faint, instead. Furthermore, he suggestedthat the outer region slowly moving object in the sky.l After they followed it for could be the source for the comets that periodically come See a few days, they realized that they had found the frrst ob- blazing through the inner Solar System. When Kuiper for- ject beyond Neptune since Clyde Tombaugh discovered mulated his argument, Pluto was thought to be massive mry Pluto in 1930. enough not to fall below the straight interpolating line in Today, more than 800 additional members of what is figu.re 2. Nowadays, astronomers regard the Kuiper belt Hu- now known as the Kuiper belt have been discoveredin the as lying beyond the orbit of Neptune. lork outer Solar System.2Their existence is forcing a change in Almost four decades after Kuiper's analysis, Martin Duncan, Thomas Quinn, and Scott Tremaine, all then of ence the University of Toronto, took advantage of the growing T power of computers to simulate the long-term gravita- April2004 PhysicsToday 49 @ 2004 American Instituteol Physics,S-0031-9228-O4O4-O2O-7 gleater numbers of objects tional influence of planets and showed that one class of a factor of 100 or more. As it became clear that the comets-the Jupitei-family comets-were best explained began to be detected, however, be- by the existenceofa band ofsmall bodiesjust beyond Nep- tro--b"" of Kuiper belt objects decreasesdramatically is the tune's orbit. They named that group of bodies the Kuiper yond about 43 AU. (An astronomical unit, or AU, belt.3 Jewitt and-Luu found the first object in the hypoth- mean distance from the Sun to Earth; Pluto, for instance, esized Kuiper beltjust five years later, in 1992' is on average 39.5 AU from the Sun.) For many years, arguments circulated that the drop- The edgeof the SolarSystem? off was a simpie consequenceof more distant Kuiper belt One of the frrst surprises after the discovery of the Kuip-er objects'beingmuch fainter. After all, t}rreUr2 decreasein belt was that it appea"s to contain only about IVo of the light intensity happens twice for Kuiper belt objects----once mass needed to make up for the deficit noted by Kuiper' oi th" way fiom the Sun to the object and again on the Even more interesting, early studies by Alan Stern at the way back to Earth. Thus, an object at 50-AU is only Southwest Research Institute in Boulder, Colorado, sug- (40i50)4= 0.41 times as bright as one at 40 AU. But three gested that the Kuiper belt did not even contain enough years ago, separate analyses by Lynne Allen (then at the the largest irass to have formeditself.a That is, building up University of Michigan' Ann Arbor) and colleagues,and by gradual of the Kuiper belt objects from the accumulation Chad Truiilo and me showed that the lack of detections which solid bodies in .-.lle" objecti-the typical way in was statistically signifrcant even when one takes into ac- to form-would have taken the Solar System are iliought count the expected dimming of distant objects.sThe true of the Solar System- longer than the age radial distribution ofKuiper belt objects has a strong peak 2 suggestsa possible resolutiot to the discrep- Figure at about 43 AU and decreases quickly beyond that' My by Stern. There one seesthat the asteroid belt "tt"y n6t"d most recent analysis rules out a resumption of Kuiper belt have an anomalously low density. The loss also appearsio densities 100 times that of the known Kuiper belt to a dis- ofthe vast majority ofthe original asteroids an expected !s tance of more than 100 AU.6 It appears that astronomers consequenceof thei" interaction with nearby Jupiter'-A have truly found the edge-or at least an edge-of our similar type of processis likely to have taken place in the solar system at about 50 AU. Many known Kuiper belt ob- outer Solar Sys-em,where an initially much more massive jects tiavel out beyond that edge, but their orbits all re- Kuiper belt was severely depleted by interaction with inner denseregion of the Kuiper belt. No known nearby Neptune. iurn to the 50 AU. An interesting potential consequenceofthe local loss objects exist exclusively beyond edge was what led Kuiper in the Kuiper belt iJthat somewherebeyond the influence The existence of an abrupt band ofplanetesimals of Neptun^e,the Kuiper belt might increase in density by to suggestthe presenceofan unseen 50 April 2004 PhYsicsTodaY http://www. physicstodaY. org 'l'-..;."... of the nebulardisk from which med may be estimatedby spreadingthe lanetinto an annulusand addinga little what wasthought to havebeen lost. As rtes,the densityfalls fairly regularlywith but one seesprecipitous density drops :roid beltand beyondNeptune's orbit. is in astronomicalunits (AU), where 1 l meandistance from the Sunto Earth. to begin with. Some astronomers have theorized that the Kuiper belt doesindeed continue outward and that its bod- ies become significantly smaller or darker, but no physi- cally plausible reason for such an abrupt change has been found. Shigeru Ida ofthe Tbkyo Institute ofTbchnology and colleagues have proposed that a close encounter with a passing star could have stripped material from the outer edge of the Solar System.TSuch an encounter would have left a clear signature in the inclinations of the remaining For many years, astronomers considered the curious Kuiper belt objects, and I have shown that this signature resonance of the orbits of Neptune and Pluto to be a fortu- is absent.8I have long speculated that one or more mod- itous coincidence that prevented close encounters between erately large planets remain to be discovered in the outer the two planets (even though their orbits cross) and so al- Solar System and that the missing mass is tied up in those lowed Pluto to suryive.

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