Transneptunian Space

We provide a nonspecialist overview of the current state of understanding of the structure and origin of our Solar System's transneptunian region (often called the Kuiper Belt), highlighting perspectives on planetesimal formation, planet migration, and the contextual relationship with protoplanetary disks. We review the dynamical features of the transneptunian populations and their associated differences in physical properties. We describe aspects of our knowledge that have advanced in the past two decades and then move on to current issues of research interest (which thus still have unclear resolution).

▪	The current transneptunian population consists of both implanted and primordial objects.
▪	The primordial (aka cold) population is a largely unaltered remnant of the population that formed in situ.
▪	The reason for the primordial cold population's current outer edge is unexplained.
▪	The large semimajor-axis population now dynamically detached from Neptune is critical for understanding the Solar System's history.
▪	Observational constraints on the number and orbits of distant objects remain poor.