1. Initial planetary material coalesced form the solar nebula. 2. Heating from accretion of planetesimals and other space debris. 3. Planetary differentiation into layers of increasingly denser material from the surface to the core. 4. Catastrophic impact and stripping away of much of the surface and the upper part of the mantle. 5. Additional accretion and formation of a rigid crust through heat loss to surrounding space.