  Recent observations (Burnes2002,Veillet2002,Margot2002a) 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 (Veillet2002): 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
(Weidenschilling2002), and one assuming large numbers of light bodies
(Goldreich2002). 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 Ward1973). 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.
