Using the density of Kuiper Belt Objects to constrain their composition and formation history

Telescopic observations of Kuiper Belt objects have enabled bulk density determinations for 18 objects. These densities vary systematically with size, perhaps suggesting systematic variations in bulk composition. We find this trend can be explained instead by variations in porosity arising from the higher pressures and warmer temperatures in larger objects. We are able to match the density of 15 of 18 KBOs within their 2 sigma errors with a constant rock mass fraction of 70%, suggesting a compositionally homogeneous, rock-rich reservoir. Because early (26)AI would have removed too much porosity in small (similar to 100 km) KBOs we find the minimum formation time to be 4 Myr after solar system formation. This suggests that coagulation, and not gravitational collapse, was the dominant mechanism for KBO formation, or the gas disk lingered in the outer solar system. We also use this model to make predictions for the density of Makemake, 2007 OR10, and MU69.