Three-Body Affairs in the Outer Solar System

TL;DR

This paper proposes a new formation mechanism for Trans-Neptunian Object binaries, explaining their high frequency, similar mass ratios, and wide eccentric orbits through three-body exchange reactions involving intermediate-mass bodies.

Contribution

It demonstrates that TNO binaries can form from a modest number of intermediate-mass bodies via three-body exchange reactions, without requiring large numbers of massive or light bodies.

Findings

TNO binaries are more frequent and have wider, more eccentric orbits than asteroid binaries.

The proposed mechanism accounts for all four distinguishing features of TNO binaries.

Three-body exchange reactions are highly efficient in the Kuiper belt environment.

Abstract

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.