The Hot Main Kuiper Belt Size Distribution from OSSOS

Using the absolute detection calibration and abundant detections of the Outer Solar System Origins Survey project, we provide population measurements for the main Kuiper Belt. For absolute magnitude H r < 8.3, there are 30,000 nonresonant main-belt objects, with twice as many hot-component objects than cold, and with total mass of 0.014 M ⊕ , only one-seventh of which is in the cold belt (assuming a cold-object albedo about half that of hot-component objects). We show that trans-Neptunian objects with 5.5 < H r < 8.3 (rough diameters 400–100 km) have indistinguishable absolute magnitude (size) distributions, regardless of being in the cold classical Kuiper Belt (thought to be primordial) or the “hot” population (believed to be implanted after having been formed elsewhere). We discuss how this result was not apparent in previous examinations of the size distribution due to the complications of fitting assumed power-law functional forms to the detections at differing depths. This shared size distribution is surprising in light of the common paradigm that the hot-population planetesimals formed in a higher density environment much closer to the Sun, in an environment that also (probably later) formed larger (dwarf planet and bigger) objects. If this paradigm is correct, our result implies that planetesimal formation was relatively insensitive to the local disk conditions and that the subsequent planet-building process in the hot population did not modify the shape of the planetesimal size distribution in this 50–300 km range.