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, we can figure out how long it takes this cubic planet to cool by figuring out how it will take the planet to radiate away its internal heat:

  Lifetime = (Amount of heat)/(Rate of heat loss)

  Lifetime ~ L x L x L / L x L = L

  The larger a planet, the longer it takes to cool.