The cold, distant outer regions of our Solar System that lie beyond Neptune's orbit have provided an exciting past decade of discoveries. The small worlds in these volumes are the remnant planetesimals of the protoplanetary disk. Their surprisingly rich variety of dynamically defined populations, on orbits that are mostly now isolated and preserved from further interaction, provide evidence for major restructuring of the giant planetary architecture in the early history of the Solar System. The mechanisms of how this happened, sharply or smoothly, are an active area of theoretical debate: no model yet provides a match to all aspects of the observed populations.
The sixteen hundred trans-Neptunian objects that are known have been discovered in the last two decades through sky surveys with optical telescopes. Upcoming surveys using wide-field imagers on 4m to 8m-class telescopes promise to draw in the details of this history. Understanding the tangled set of dynamical and physical properties in our most accessible of planetary systems will have implications for our wider understanding of the evolution of planetary systems.