The Secular Evolution of the Primordial Kuiper Belt

The Kuiper Belt is a vast swarm of comets orbiting at the Solar System’s outer edge. This Belt is comprised of debris that was left over from the epoch of planet formation, and this swarm’s distribution of orbit elements preserves a record of events that had occurred when the Solar System was still quite young. Although the common goal of most dynamical studies of the Kuiper Belt is to decipher this record, its interpretation is not entirely clear. The∼ 400 dots in Fig. 1 represent the Kuiper Belt Objects (KBOs) eccentricities e and inclinations i plotted versus their semimajor axes a. This Figure reveals the KBOs’ three major dynamical classes: the Plutinos which inhabit Neptune’s 3:2 resonance at a = 39.5 AU, the Main Belt KBOs which are the nonresonant bodies orbiting between 40∼ a∼ <48 AU, and the more distant Scattered KBOs that live in eccentric, nearly Neptune crossing orbits. The Figure also shows that the Plutinos and the Scattered KBOs have inclinations that span 0∼ i∼ <30◦, while the Main Belt KBOs appear to have a bimodal distribution of inclinations centered on i ' 2◦ and i ' 17◦ [1]. Note that accretion models show that these large ∼ 100+ km KBOs must have formed from much smaller planetesimal seeds that were initially in nearly circular and coplanar orbits having e and sin i∼ <0.001 [2]. However the swarm’s gravitational self stirring can not account for the Kuiper Belt’s current excited state, so one or more mechanisms must also have stirred up the Kuiper Belt since the time of formation.