Raphael: What line of work are you in? electricity writing literature essays alteration clergyman Current models for planet formation assemble dust in the planetary nebula pretty quickly - where "pretty quickly" means a couple of million years. When assembled at this rate, the energy from violent collisions between planetesimals (small celestial bodies thought to fuse and form planets) and the heating caused by decay of radioactive elements causes even small planetesimals to undergo large-scale or complete melting.