A Brightness Classification Boundary in the Kuiper Belt Population

The Kuiper Belt contains a large number of minor bodies that collectively constrain the dynamical history of the solar system. These bodies are often classified into distinct populations. This note identifies a well-defined boundary (at H -magnitude = 5.65) that separates bright (large) objects from dimmer (smaller) objects. The probability that the two sub-samples are drawn from the same underlying distribution is p ∼ 10 −15 . The larger bodies (estimated radius R ≳ 125 km) are dynamically hot, as measured by their locations in the e - sin i plane, and likely represent an implanted population. The smaller bodies ( R ≲ 125 km) are dynamically colder and are likely to be primordial. The bright/large/hot population currently contains ∼200 members, including all ∼50 dwarf planet candidates in the Kuiper Belt.