| The Kuiper belt, the population of small icy planetesimals orbiting the Sun beyond Neptune, offers an exciting new avenue for studying the formation and evolution of the Solar System. Over 500 Kuiper belt objects (KBOs) have been detected in the decade since the first KBO was discovered. Almost all of these have been discovered within 50 AU of the Sun, despite surveys sensitive enough to detect objects beyond 60 AU. This lack of distant KBOs contradicts the current model of the Kuiper belt, which predicts a dynamically cold and primordially dense population of KBOs beyond 50 AU.; We have conducted a deep survey (R ∼ 25) for KBOs over a total of 2.3 deg2. In order to reach these faint limiting magnitudes for moving objects, we have developed software to rapidly shift and add a series of images at all potential KBO motion vectors. R = 25 is faint enough to detect KBOs of diameter D 200 km at a distance of 60 AU. Our survey discovered 34 objects; none of these objects, however, is beyond 53 AU.; The first group of fields observed in this survey, with the deepest limiting magnitudes, find the mean volume density of D > 160 km KBOs in the 55–65 AU region is (at the >95% confidence level) lower than the mean density in the 30–50 AU region, and at most ⅔ of the mean density from 40–50 AU, if the distant Kuiper belt is of similar vertical dispersion as the classical KBOs.; One theory proposed to explain the observed absence of KBOs beyond 50 AU suggests the outer Kuiper belt is in an extremely cold, primordial dynamical state, visible only as a very thin disk which could have slipped between previous deep survey fields. The second stage of our survey was designed to search the most likely locations for this thin disk. We find that we can exclude the existence of a distant disk thinner than σ = , inclined by i ≤ 1° to the invariable plane and containing more than 1.2 times as many D > 185 km KBOs between 50 and 60 AU as the observed inner Kuiper belt at the 95% CL. |
