ScienceWeek

PLANETARY SCIENCE: NEPTUNE AND THE EARLY SOLAR SYSTEM

The following points are made by Alessandro Morbidelli (Science 2004 306:1302):

1) Planetary researchers are finding increasing evidence that the orbital separations between the giant planets increased substantially as a result of interactions between the planets and a disk of "planetesimals" that were left over after planet formation. The evidence comes from the Kuiper belt, a population of small (diameter less than 1000 km) bodies at the outer edge of today's Solar System that are the last remnants of this disk.

2) Twenty years ago, computer simulations [1] demonstrated that planetary orbits should expand or contract to conserve energy and angular momentum while the planets eject the planetesimals left over from planet formation from their neighborhoods. A decade later, Malhotra [2] proposed that the orbits of Pluto and the "Plutinos" -- a subpopulation of the Kuiper belt -- were probably caused by Neptune's migration. She concluded that Neptune, which is now at 30 astronomical units (AU; 1 AU is the mean distance from the Earth to the Sun), has moved outward by at least 7 AU since its formation.

3) The orbital period of the Plutinos is 1.5 times that of Neptune, a behavior referred to as a 2:3 resonance. As the orbit of Neptune expanded, the orbital period of the planet increased. Hence, the location of the 2:3 resonance with Neptune also moved outward through the planetesimal disk. Malhotra [2] demonstrated that when planetesimals were swept by the resonance like house dust by a broom, they were likely to be "trapped" in resonance. Trapped planetesimals then moved outward with the resonance, while the ellipticity of their orbits slowly increased. In contrast, untrapped planetesimals kept their original orbital radius and small orbital ellipticity and inclination. According to this picture, the current Plutinos are the trapped planetesimals.

4) However, the study was not definitive. The proportions of Plutinos with high and low orbital inclinations could not be reproduced in Malhotra's migration model [3]. Furthermore, important properties of the Kuiper belt, such as its lower-than-expected mass [4], and the two types ("hot" and "cold") of Kuiper belt objects other than the Plutinos [5] could not be explained solely by the migration model. They seemed to suggest that some other mechanism sculpted the Kuiper belt. A series of more sophisticated models of the migration process now provide confirmation of the original idea, showing that all the main properties of the Kuiper belt can be explained by planet migration alone. But for this to be possible, the planetesimal disk through which the migration took place must not extend beyond about 30 AU.

References (abridged):

1. J. Fernandez, W. Ip, Planet. Space Sci. 44, 431 (1984)

2. R. Malhotra, Astron. J. 110, 420 (1995)

3. R. S. Gomes, Astron. J. 120, 2695 (2000)

4. The current mass of the Kuiper belt is only 0.01 to 0.1 Earth masses, but a few tens of Earth masses had to exist in the region where the Kuiper belt bodies formed in order for the growth time scale to be short enough. Therefore, the current Kuiper belt apparently contains only 0.1 to 1.0% of its primordial mass.

5. The inclination distribution of the nonresonant Kuiper belt objects is bimodal. A first component, called "cold population", has inclinations smaller than about 4 deg. A second component, called "hot population", has a much broader inclination distribution, extending up to 30 deg to 40 deg. The hot and cold populations seem to have different physical properties.

Science http://www.sciencemag.org

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ASTRONOMY: ON THE KUIPER BELT

The following points are made by Michael E. Brown (Physics Today 2004 April):

1) Astronomers studying the formation and evolution of the outer Solar System have had, until recently, few data points around which to spin their theories. The outer Solar System's four giant planets were each created and moved around by a complex suite of interactions. Trying to piece together all of those interactions to discern a history would be like trying to use just four large shipwrecks washed ashore to understand the flows of all of the oceans' currents. The breakthrough for astronomers came in November 1992. After a long search, Jewitt and Luu (1) found a single faint, slowly moving object in the sky. After they followed it for a few days, they realized that they had found the first object beyond Neptune since Clyde Tombaugh (1906-1997) discovered Pluto in 1930.

2) Today, more than 800 additional members of what is now known as the "Kuiper belt" have been discovered in the outer Solar System.(2) Their existence is forcing a change in the picture that astronomers held just a decade ago of the outer Solar System's dynamical evolution. In many ways, Kuiper belt objects behave like test particles that trace the gravitational effects of the giant planets' rearrangements and perturbations. The hundreds of such objects strewn throughout the outer Solar System give concrete data that allow astronomers to trace the history of that system.

3) The prediction of a belt of small bodies, or planetesimals, beyond the orbit of Neptune was made in 1950 by Gerard Kuiper (1905-1973), who used 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 its entire mass into an annulus centered around the orbit of the giant planet. That annulus represented the region of the nebula that went into making Jupiter. Kuiper knew, however, that some material initially in that part of the nebula must have been lost, because Jupiter has a higher abundance of elements heavier than hydrogen and helium than the Sun does. So he added a little more mass to the annulus to make up for the lost hydrogen and helium and bring that region of the theoretical nebula to solar composition. Kuiper applied the same procedure to the remaining giant planets and to the terrestrial planets. (The terrestrial planets have lost almost all of their hydrogen and helium so a large amount of extra material had to be added in.) His method yields an approximate reconstruction of the mass distribution of the initial nebula.

4) Kuiper noted that the surface density of the nebula smoothly dropped from the inside to the outside until, beyond Pluto, the density plummeted. He reasoned that the nebula should not have an abrupt edge and that beyond Pluto was a realm where densities were never high enough to form large bodies, but where small icy objects existed instead. Furthermore, he suggested that the outer region could be the source for the comets that periodically come blazing through the inner Solar System.

5) In 1987, almost four decades after Kuiper's analysis, Duncan et al (3) took advantage of the growing power of computers to simulate the long-term gravitational influence of planets and showed that one class of comets -- the Jupiter-family comets --were best explained by the existence of a band of small bodies just beyond Neptune's orbit. They named that group of bodies the "Kuiper belt". Jewitt and Luu found the first object in the hypothesized Kuiper belt just five years later, in 1992.(1,4,5)

References (abridged):

1. D. Jewitt, J. Luu, Nature 362, 730 (1993)

2. See the list updated daily at http://cfa-www.harvard.edu/iau/Ephemerides/Distant/index.html

3. M. Duncan, T. Quinn, S. Tremaine, Astron. J. 94, 1330 (1987)

4. S. A. Stern, Astron. J. 110, 856 (1995)

5. R. L. Allen, G. M. Bernstein, R. Malhotra, Astrophys. J. 549, L241 (2001); C. A. Trujillo, M. E. Brown, Astrophys. J. 554, L95 (2001)

Physics Today http://www.physicstoday.org

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PLANETARY SCIENCE: ON BINARIES IN THE KUIPER BELT

The following points are made by Joseph A. Burns (Nature 2004 427:494):

1) In the frigid outskirts of the Solar System, beyond the orbit of Neptune, lies the Kuiper belt. This doughnut-shaped region contains 100,000 frozen bodies, each more than 200 kilometers across. It is scarcely a decade since the first Kuiper-belt object (KBO) was sighted, but now the positions and orbits of nearly 800 are known. Sizes have been measured for a handful and, for scores more, crude colors have been detected. A few KBOs have been found to be binary -- that is, two objects looping around each other while, as a pair, they circle the Sun.

2) When Pluto's satellite Charon was discovered in 1978, these two bodies became the first identified binary in the Kuiper belt -- although this was well before the belt itself was recognized as a distinct province of our Solar System. The next pair was not found until 2001, but now a dozen binary KBOs have been pinpointed, roughly half of them by ground-based observatories and the others by the Hubble Space Telescope.

3) Kuiper-belt couples(1,2) seem strikingly different from other binaries(3) in the Solar System. Among the known KBO pairs, the components are roughly equal in size and move on highly elongated orbits around each other, typically separated by a distance hundreds to thousands of times greater than their radii. In contrast, binary systems in the main asteroid belt, between Mars and Jupiter, are usually made up of tiny companions that are tightly bound in circular orbits around their much larger partner. The abundance and orbital configurations of the known KBO binaries, as well as the relative sizes of the individuals themselves, provide potentially pivotal clues to how KBO binaries originated and, more generally, to how bodies accumulate and evolve in the outer Solar System.

4) Like the solid planets themselves, individual KBOs are believed to have formed in the early Solar System through the agglomeration of smaller bodies. So how do any KBOs end up travelling in tandem? Cataclysmic collisions probably led to the creation of Earth's Moon, the Pluto-Charon system and many asteroid companions(4), but the progeny of such events are born too close together and are too different in size to account for the Kuiper-belt binaries. Furthermore, each of the Kuiper-belt systems contains more angular momentum than could have been transferred in any collision that did not totally destroy and disperse the colliding bodies. The formation rate through collisions alone is, anyway, much too low to account for the total binary population of KBOs(5).

5) Debris ejected in a two-body collision could, however, have been captured to yield a broad binary system had there been a third body nearby. Alternatively, two passing bodies can undergo permanent mutual capture, as long as their relative energies are lowered sufficiently through the dynamical drag of small background objects or by the gravitational scattering of another large passing KBO. This scheme(4) should also generate many small, distant companions. But to produce a reasonable yield of binaries, these mechanisms, require either many more primordial building-blocks or relatively greater numbers of small bodies than expected.

6) The theory of Funato et al(1) combines aspects of both of these mechanisms. They suggest that objects of disparate masses initially form a close binary system through gravitational instability or through collisions. Then the pair encounters another KBO that typically has a mass similar to that of the larger of the companions. Following an elaborate gravitational dance, the two massive objects pair up and the small object is ejected from the system. This idea produces binaries whose attributes match those of the observed binary population of the Kuiper belt.

References (abridged):

1. Funato, Y., Makino, J., Hut, P., Kokubo, E. & Kinoshita, D. Nature 427, 518-520 (2004)

2. Noll, K. Earth Moon Planets (in the press)

3. Merline, W. J. et al. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 289-312 (Univ. Arizona Press, Tucson, 2003)

4. Michel, P., Benz, W., Tanga, P. & Richardson, D. C. Science 294, 1696-1699 (2001)

5. Stern, S. A. Astron. J. 124, 2300-2304 (2002)

Nature http://www.nature.com/nature

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