Ample of information arrives from the Kepler space mission, and from radial velocity measurements of nearby stars on the planets that orbit them. More than two thousand planet and planet candidates are known with diverse properties. Together with observations of disk around young stars and of small bodies in our own solar system, provide an opportunity to sharpen theories of planet formation. We show that it is likely that shattering of colliding solid bodies is an essential process in fast planet formation. We discuss how a collisional cascade evolves and how it could be studied today in the Kuiper Belt. We report observations of the only two sub-km objects there, and confront the findings with these theories.