The Kuiper Belt and the Primordial Evolution of the Solar System

We discuss the structure of the Kuiper belt as it can be inferred from the first decade of observations. In particular, we focus on its most intriguing properties — the mass deficit, the inclination distribution, and the apparent existence of an outer edge and a correlation among inclinations, colors, and sizes — which clearly show that the belt has lost the pristine structure of a dynamically cold protoplanetary disk. Understanding how the Kuiper belt acquired its present structure will provide insight into the formation of the outer planetary system and its early evolution. We critically review the scenarios that have been proposed so far for the primordial sculpting of the belt. None of them can explain in a single model all the observed properties; the real history of the Kuiper belt probably requires a combination of some of the proposed mechanisms.