This is a response to a question in Scientific American given by Dr. Quentin Williams who specializes in the deep interior. I searched "Why is the earth's core so hot" on Duckduckgo. (I'm not sure if current posting guidelines allows me to give links to periodicals?)
I've been wondering about this question for some time now. There are the following two contributors:
1. Heat left over from the planet's initial formation. In the early 19th century Lord Kelvin estimated the temperature based on a homogenous sphere of uniform initial temperature...