Planetary Cooling

• Heat flows from the center to the surface in a planet and is lost from the surface. To get a feel for how this depends upon the size of the planet, consider the simple picture:
  

  So, the time it takes for this cubic planet to cool is roughly given by

  Cooling Time = (stored heat)/(rate of heat loss) ~ L³/L² = L

  The larger the planet, the longer it takes to cool. That is, we predict that planets like the Earth and Venus should have warm interiors while smaller objects like the Moon and Mercury should have solid (or, at best, plastic) interiors.