On the Size Dependence of the Inclination Distribution of the Main Kuiper Belt

We present a new analysis of the currently available orbital elements for the known Kuiper belt objects. In the nonresonant, main Kuiper belt, we find a statistically significant relationship between an object's absolute magnitude (H) and its inclination (i). Objects with H < 6.5 (i.e., radii ≳170 km for a 4% albedo) have higher inclinations than those with H > 6.5 (radii ≲170 km). We have shown that this relationship is not caused by any obvious observational bias. We argue that the main Kuiper belt consists of the superposition of two distinct distributions. One is dynamically hot, with inclinations as large as ∼35° and absolute magnitudes as bright as 4.5; the other is dynamically cold, with i ≲ 5° and H > 6.5. The dynamically cold population is most likely dynamically primordial. We speculate on the potential causes of this relationship.