What did the earliest bodies in our Solar System look like, and what was their fate? It’s difficult to tell, because it’s not clear that there are any of them left. Lots of the earliest material was swept up into the planets. Many of the smaller bodies that remained are products of multiple collisions and have perhaps formed and re-formed multiple times—some are little more than rubble piles barely held together by gravity.

Without some knowledge of what these bodies looked like, then, it’s difficult to determine whether our models of the physics of the early Solar System are right and whether similar processes are likely to be in play in exosolar systems.

Now, some researchers have found a way to infer the sizes of objects present in the early Solar System: looking at the craters they left behind when they smashed into Pluto and Charon. The results suggest a shortage of objects smaller than 2km in diameter and suggest that much of the material in the Kuiper Belt was quickly swept up into larger objects, which somehow avoided smashing into each other and liberating a new generation of smaller fragments.