Introduction

The Classical Kuiper Belt has an edge at about 47 AU. It may have been carved by tidal truncation or represent the outer limits to which objects were pushed by Neptune's migration.

Detection of the Edge

"The observed radial distribution of KBOs is deficient in objects beyond about 50 AU, relative to simulations of power-law Kuiper Belt disks. We have no firm explanation of this observation. The Kuiper Belt might be truncated at about this distance, there might be a steep decrease in the maximum size of KBOs at larger heliocentric distances, or some combination of these two effects might conspire to yield the observed radial distribution." Jewitt et al. 1998

Later, the edge was confirmed in a smaller but deeper survey by Allen et al. (2001) and in a larger and deeper survey by Trujillo et al. (2001).

The Cause of the Edge

The edge, like many other features of the Kuiper Belt, was not expected or predicted by any prevailing theories. Perhaps for this reason, its reality was accepted only slowly by many, but it now is generally acknowledged as real. [Doubters asserted that the edge is an illusion, caused by the faintness of more distant objects. This possibility was rejected in the 1998 paper based on numerical simulations of the data which show that we could see more distant objects if they were present.]

The first group to attempt an explanation invoked tidal truncation of the Kuiper Belt by a passing star (Ida et al (2000): Ap. J. 528, 351-356). To cut the disk at 50 AU, the star would have to pass about 150 AU from the sun. Such a close encounter is highly improbable in the present environment of the sun (the mean distance between stars is near 1 parsec, or 200,000 AU). But if the sun formed in a dense cluster, then perhaps such an encounter could have occurred.

Another explanation could be that the edge of the Classical Belt simply marks the distance out to which objects were transported after being swept along by the migrating Neptune 3:2 resonance, which now coincides with the outer edge.

Still another, as noted above, is that the edge reflects a deficiency only in the largest, most easily observed objects, but that smaller, fainter objects exist beyond it. This possibility has grown weaker since 1998, as new surveys of greater depth and area have failed to find any more distant Classical KBOs.

Beyond the Edge

What is beyond the edge? We know that the scattered KBOs travel far beyond the edge - to 1000's of AU, in fact. But could there also be a distant population of bodies on near-circular orbits, like a continuation of the Classical Belt but at larger distances? Observationally, the answer is a clear "yes", provided the outer belt does not begin until 100 AU or more from the sun. Because the brightness of objects viewed in reflected sunlight varies as a steep (inverse 4th) power of distance, this possibility will be difficult to refute. A better constraint is expected from the occultation technique , in which KBOs can potentially be detected from their shadows.

David Jewitt

Kuiper Belt

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