*OBSERVATORY*


    The Kuiper Waltz

*By HENRY FOUNTAIN*

Published: February 10, 2004

The Kuiper Waltz

The early solar system can be likened to a crowd of dancers doing the
boogaloo in a nightclub ? with lots of whirling, flailing and flinging
and even some pushing and shoving before things finally settled down.

There was also a lot of cutting in on partners, at least on the fringes
of the cosmic dance floor. New research suggests that pairs of like-size
bodies in the solar system's outer reaches were formed when a third
object broke up an existing pair.

The bodies are in the Kuiper Belt, that region beyond Neptune populated
with perhaps 100,000 relatively small objects (including, by most
classifications, Pluto, which is the largest of the lot). A small
percentage are binary pairs that orbit each other.

Elsewhere in the solar system, in most binaries ? pairs of asteroids,
for example ? one object is much larger than the other, the two are
close together and the orbit is circular. In the Kuiper Belt, however,
the bodies are of similar size and the orbits are wide and elliptical.

Writing in the journal Nature, scientists from Japan, Taiwan and the
United States have described a mechanism to explain the peculiarities of
these pairs.

The process, the scientists' simulations suggest, occurs in two stages.
The gravitational force of a relatively large body captures a smaller
one that passes close by, or even hits, the first. At a later point, a
third object, similar in size to the larger of the two, happens by. The
interloper cuts in, and all three dance together for a while. But
eventually the interacting gravitational forces kick the small body out
and the two remaining bodies waltz off together.