This is a drawing of the evolution of the interior of a giant planet.
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Image from: The New Solar System

How the Interior of Neptune Formed

The drawing shows a possible path in the evolution of the interior of giant planets. As the planets drew material from the solar nebula, bits of heavy, solid material accumulated inside the forming planet, as illustrated in figure A.

Once the planet finished forming, these heavy, solid bits of material fell into the center of the planet, as illustrated in figure B. The heavy elements of the planet made up a much smaller portion of the planet than the gaseous elements. The amount of gas and ice which make up Neptune depended upon where Neptune was in the primordial cloud.

Eventually, the heavy, solid material at the center solidified into a core, as illustrated in figure C.

Leftover heat from this process of Neptune's forming may still influence the motions in Neptune's atmosphere.


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