In the last few years, researchers have discovered more than 500 objects in the Kuiper belt, a gigantic outer ring in the outskirts of the solar system, beyond the orbit of Neptune. Of these, seven so far have turned out to be binaries--two objects that orbit each other. The surprise is that these binaries all seem to be pairs of widely separated objects of similar size. This is surprising because more familiar pairings, such as the Earth/moon system, tend to be unequal in size and/or rather close together.

To account for these oddities, scientists from the California Institute of Technology have devised a theory of Kuiper belt binary formation. Their work is published in the December 12 issue of the journal Nature.

According to Re'em Sari, a senior research fellow at Caltech, the theory will be tested in the near future as additional observations of Kuiper belt objects are obtained and additional binaries are discovered. The other authors of the paper are Peter Goldreich, DuBridge Professor of Astrophysics and Planetary Physics at Caltech; and Yoram Lithwick, now a postdoc at UC Berkeley.

"The binaries we are more familiar with, like the Earth/moon system, resulted from collisions that ejected material," says Sari. "That material coalesced to form the smaller body. Then the interaction between the spin of the larger body and the orbit of the smaller body caused them to move farther and farther apart."

"This doesn't work for the Kuiper belt binaries," Sari says. "They are too far away from each other to have ever had enough spin for this effect to take place." The members of the seven binaries are about 100 kilometers in radius, but 10,000 to 100,000 kilometers from each other. Thus their separations are 100 to 1,000 times their radii. By contrast, Earth is about 400,000 kilometers from the moon, and about 6,000 kilometers in radius. Even at a distance of 60 times the radius of Earth, the tidal mechanism works only because the moon is so much less massive than Earth.

Sari and his colleagues think the explanation is that the Kuiper belt bodies tend to get closer together as time goes on -- exactly the reverse of the situation with the planets and their satellites, where the separations tend to increase. "The Earth/moon system evolves 'inside-out', but the Kuiper belt binaries evolved 'outside-in,'" explains Sari.

Individual objects in the Kuiper belt are thought to have formed in the early solar system by accretion of smaller objects. The region where the gravitational influence of a body dominates over the tidal forces of the sun is known as its Hill sphere. For a 100-kilometer body located in the Kuiper belt, this extends to about a million kilometers. Large bodies can accidentally pass through one another's Hill spheres. Such encounters last a couple of centuries and, if no additional process is involved, the "transient binary" dissolves, and the two objects continue on separate orbits around the sun. The transient binary must lose energy to become bound. The researchers estimate that in about 1 in 300 encounters, a third large body would have absorbed some of the energy and left a bound binary. An additional mechanism for energy loss is gravitational interaction with the sea of small bodies from which the large bodies were accreting. This interaction slows down the large bodies. Once in every 30 encounters, they slowed down sufficiently to become bound.

Starting with a binary of large separation a million kilometers apart, continued interaction with the sea of small objects would have led to additional loss of energy, tightening the binary. The time required for the formation of individual objects is sufficient for a binary orbit to shrink all the way to contact. Indeed, the research predicts that most binaries coalesced in this manner or at least became very tight. But if the binary system was formed relatively late, close to the time that accretion in the Kuiper belt ceased, a widely separated binary would survive. These are the objects we observe today. By this mechanism it can be predicted that about 5 percent of objects remain with large enough separation to be observed as a binary. The prediction is in agreement with recent surveys conducted by Caltech associate professor of planetary astronomy Mike Brown. The majority of objects ended up as tighter binaries. Their images cannot be distinguished from those of isolated objects when observed from Earth using existing instruments.

These ideas will be more thoroughly tested as additional objects are discovered and further data is collected. Further theoretical work could predict how the inclination of a binary orbit, relative to the plane of the solar system, evolves as the orbit shrinks. If it increases, this would suggest that the Pluto/Charon system, although tight, was also formed by the 'outside-in' mechanism, since it is known to have large inclination.