Wikipedia says "Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[" I don't see where the rock comes from. In my mind, rock can be formed from dust only by application to lots of heat and pressure. How did Kupier Belt objects get exposed to heat and pressure? Or is there another mechanism to form rock from dust? Or did the rocky bits form in the inner parts of the disc and get ejected to the outer parts?