This is a drawing of the evolution of the interior of a giant planet.
Click on image for full size
Image from: The New Solar System
How the Interior of Saturn Formed
The drawing shows how the inside of giant planets may have formed.
As the planets drew material from the solar cloud, bits of heavy rock came to where the planet was forming, as shown in figure A.
Once the planet finished forming, these heavy bits of rock fell into the middle of the planet, as illustrated in figure B.
As shown in the picture, the gas part of the planet is much bigger than the rocky part. Saturn drew a large amount of gas to itself from the nebula because of where Saturn was in the original cloud.
Eventually, the heavy, rocky material at the center became a core, as illustrated in figure C.
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