Observational limits on a distant cold Kuiper belt

Almost all the more than 500 known Kuiper belt objects (KBOs) have been discovered within 50 AU of the Sun. One possible explanation for the observed lack of KBOs beyond 50 AU is that the distant Kuiper belt is dynamically very cold and thus thin enough on the sky to have slipped between previous deep survey fields. We have completed a survey designed to search for a dynamically cold distant Kuiper belt near the invariable plane of the solar system. In 2.3 deg² we have discovered a total of 33 KBOs and 1 Centaur, but no objects in circular orbits beyond 50 AU. We find that we can exclude at 95% CL the existence of a distant disk 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, if the distant disk is thinner than σ = 1°.75.