OSSOS Finds an Exponential Cutoff in the Size Distribution of the Cold Classical Kuiper Belt

The cold main classical Kuiper Belt consists of the nonresonant small solar system bodies with low orbital inclinations and orbital semimajor axes between 42.4 and 47.7 au. These objects likely formed in situ, and the population has experienced minimal collisional modification since formation. Using the Outer Solar System Origins Survey ensemble sample and characterization, combined with constraints from deeper surveys and supported by evidence from the Minor Planet Center catalog and the Deep Ecliptic Survey, we determine the absolute magnitude H ( r ) distribution of the cold classical belt from H ( r ) similar or equal to 5 to 12 (roughly diameters of 400-20 km). We conclude that the cold population's H ( r ) distribution exhibits an exponential cutoff at large sizes. Exponential cutoffs at large sizes are not a natural outcome of pairwise particle accretion, but exponentially tapered power-law size distributions are a feature of numerical simulations of planetesimal formation via a streaming instability. Our observation of an exponential cutoff agrees with previous observational inferences that no large objects (D greater than or similar to 400 km) exist in the cold population. We note that the asymptotic slope of the H ( r ) distribution is consistent with alpha similar to 0.4. This asymptotic slope is also found in streaming instability modeling of planetesimal formation and is thus not necessarily associated with achieving collisional equilibrium. Studies of the trans-Neptunian region are providing the parameters that will enable future streaming-instability studies to determine the initial conditions of planetesimal formation in the approximate to 45 au region of the Sun's protoplanetary disk.

Automated summary

The study shows that there is a lack of large objects (diameter greater than or equal to 400 km) in the cold classical belt, with an exponential cutoff in the H(r) distribution at large sizes, consistent with numerical simulations.