This is an infrared image of Jupiter showing regions which are hot.
Click on image for full size
NASA
The Source of Heat from Within Neptune
The picture shows places on Jupiter which are hot. Jupiter is a very warm place, as shown in the picture, sort of like a warm log in the fireplace which gives off heat.
Like Jupiter, Neptune also gives off a lot of heat.
There are several ways for a heavenly object to make energy from inside. The first is by fusion, the way the sun makes energy. Another way is with radioactive material deep within the ground, the way a planet makes energy.
For the giant planets, the way that works is the mere fact that energy is made when a planet is in the process of shrinking together.
The fact that Neptune is still shrinking together shows that the process of forming is still going on.
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