Eli5: How does the core of the Earth (or any planets) not cool and still give off tons of heat energy? Where is this energy coming from?
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Planets do cool down over time. However, there are a lot of factors that determine how long that takes. One of the biggest is size. The larger a planet is,the longer it takes to cool down for two major reasons. The first is that the amount of thermal energy within the planet is proportional to the volume of the planet (i.e. the amount of “stuff” in the planet). The second is that the amount of heat loss is proportional to the surface area of the planet. Volume grows by the radius of the planet cubed, while surface area grows by the radius of the planet squared. This means that as a planet grows in size, the amount of thermal energy inside of it grows faster than the amount of thermal energy it loses, so it will take longer to cool off.
Also, planets get their initial heat from their formation. As all the rocks and metals collide with each other to form the planet, they create a lot of heat, this heat makes the planet very hot to begin with, however it wouldn’t last billions of years on its own. The only reason earth and other planets like Venus haven’t cooled off is because there are many radioactive elements within them that decay over time and create a lot of residual heat that keeps the planets warm far longer than they otherwise would stay warm.
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