However, the very low densities of smaller KBOs are hard to explain without assuming that the bodies have a high degree of porosity. Porosity is a known factor in the formation of asteroids throughout the solar system - gaps throughout the structure of rocky bodies less than 350 kilometers in diameter are thought to lower the overall density. Asteroids over 350 kilometers become so massive that porosity decreases; the gravitational compression pulls the material closer together, reducing porosity and increasing density.
According to Brown, this porosity transition should occur in KBOs larger than 350 kilometers wide. But as 2002 UX25 shows, this transition hasn't happened up to a size of 650 kilometers. This factor creates a problem. If larger KBOs over 1,000 kilometers (620 miles) formed through the coalescence of smaller KBOs (like 2002 UX25), it isn't possible that large rock-rich KBOs could have such high densities.
In the case of an object the size of Eris, for example, with a measured density of 2.5 g/cm3, even with the gravitational compression exerted by the 2,326 kilometer-wide dwarf planet, the low density, high porosity material from an objects like 2002 UX25 cannot be compressed to such a high degree. Such an object "would still have a density close to 1 g/cm3 rather than the 2.5 g/cm3 density of Eris," writes Brown. On this evidence alone, large KBOs cannot form through agglomeration of many small KBOs like 2002 UX25.