Moving animations available at http://pr.caltech.edu/media/kuiper/
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.
