Once a terrestrial planet had formed from a large number of planetesimals, additional thermal energy from could have melted it and allowed it to differentiate into a dense metallic core and a lower density crust.

radioactive decay

Explanation:

Radioactive decay occurs as the nucleus of a set of unstable isotopes is disrupted as a result of great atomic instability. The nuclear breakdown releases a large amount of thermal energy, which has a very high temperature. An example of radioactive decay occurs when terrestrial planets form through a group of planetesimals. The thermal energy released in this process can even melt this planet.

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