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I just saw a show about the theorized "Planet 9." One possibility is that it's an ice planet. But it could have a liquid water core from its gravitational energy crushing in the core and making it heat up. Where does the energy for that heat come from? If the gravitational energy comes from the planet's mass, shouldn't the energy be constant? Unless some other part of the process make the planet lose mass.

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There are two sources of the heat of a planet's core:

There is the original potential energy of the asteroids that fell together to form the planet.

The material of the core is such a good insulator, that the small amount of radioactivity that occurs naturally in the core material is enough to be the input of energy that keeps the core molten after the planet has formed.

See

https://www.scientificamerican.com/article/why-is-the-earths-core-so/

http://phys.org/news/2006-03-probing-earth-core.html

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