The Edge of the Solar System

The population of the Kuiper Belt within 50 AU of the Sun has likely been severely depleted by gravitational perturbations from the giant planets, particularly Neptune. The density of Kuiper Belt objects is expected to be two orders of magnitude higher just beyond 50 AU, where planetary perturbations are insignificant. In 1998 and 1999, we surveyed for Kuiper Belt Objects (KBOs) in 6 fields of the ecliptic (total sky area 1.5 deg^2) to limiting magnitudes between R=24.9 and R=25.9. This is deep enough to detect KBOs of diameter >~ 160 km at a distance of 65 AU. We detected 24 objects. None of these objects, however, is beyond 53 AU. Our survey places a 95% CL upper limit of Sigma<5 deg^-2 on the surface density of KBOs larger than ~160 km beyond 55 AU. This can be compared to the surface density of ~6 deg^-2 of 160 km KBOs at distances 30--50 AU determined from this survey and previous shallower surveys. The mean volume density of D>160 km KBOs in the 55--65 AU region is, at >95% confidence, less than the mean density in the 30--50 AU region, and at most twothirds of the mean density from 40--50 AU. Thus, a substantial density increase beyond 50 AU is excluded in this model-independent estimate, implying that some process or event in the history of the Solar System has truncated the distribution of 160-km planetesimals at ~50 AU. A dense primordial disk could be present beyond 50 AU if it contains only smaller objects, or is sufficiently thin and inclined to have escaped detection in our 6 survey fields.