  Recent observations (Burnes et al 2002,Veillet et al 2002, Margot et al
2002a) have revealed an unexpectedly high binary fraction among the
Trans-Neptunian Objects (TNOs) that populate the Kuiper Belt. The TNO binaries
are strikingly different from asteroid binaries in four respects (Veillet et al
2002): their frequency is an order of magnitude larger, the mass ratio of their
components is closer to unity, and their orbits are wider and highly eccentric.
Two explanations have been proposed for their formation, one assuming large
numbers of massive bodies (Weidenschilling 2002), and one assuming large
numbers of light bodies (Goldreich et al 2002). We argue that both assumptions
are unwarranted, and we show how TNO binaries can be produced from a modest
number of intermediate-mass bodies of the type predicted by the gravitational
instability theory for the formation of planetesimals (Goldreich and Ward
1973). We start with a TNO binary population similar to the asteroid binary
population, but subsequently modified by three-body exchange reactions, a
process that is far more efficient in the Kuiper belt, because of the much
smaller tidal perturbations by the Sun. Our mechanism can naturally account for
all four characteristics that distinguish TNO binaries from main-belt asteroid
binaries.
