  As to April 2010, 48 TNO (trans-Neptunian Object) binaries have been found.
This is about 6% of known TNOs. However, in previous theoretical studies of
planetary formation in the TNO region, the effect of binary formation has been
neglected. TNO binaries can be formed through a variety of mechanisms, such as
three-body process, dynamical friction on two massive bodies, inelastic
collisions between two bodies etc. Most of these mechanisms become more
effective as the distance from the Sun increases. In this paper, we studied
three-body process using direct N-body simulations. We systematically changed
the distance from the Sun, the number density of planetesimals, and the radius
of the planetesimals and studied the effect of the binaries on the collision
rate of planetesimals. In the TNO region, binaries are involved in 1/3 - 1/2 of
collisions, and the collision rate is increased by about a factor of a few
compared to the theoretical estimate for the direct two-body collisions. Thus,
it is possible that the binaries formed through three-body process
significantly enhance the collision rate and reduce the growth time scale. In
the terrestrial planet region, binaries are less important, because the ratio
between the Hill radius and physical size of the planetesimals is relatively
small. Although the time scale of our simulations is short, they clearly
demonstrated that the accretion process in the TNO region is quite different
from that in the terrestrial planet region. Simulations which cover longer time
scale are required to obtain more accurate estimate for the accretion
enhancement.
