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 Uranus Formed
The drawing shows a possible history of the inside of giant planets.
As the planets drew material from the solar cloud, bits of heavy rock collected inside the forming planet, 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. That is because the amount of gas and ice which came to Uranus in the beginning depended upon where Uranus was in the original solar cloud.
Eventually, the heavy, rocky material at the center became a core, as illustrated in figure C.
Leftover heat from this process of Uranus' forming may still influence the motions in Uranus' atmosphere.
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