This is an image of Jupiter.
Click on image for full size
Courtesy of NASA
The giant planets have definitely changed since their formation. But how much remains to be seen.
Most of the original air of the giant planets remains in place. (The earth-like planets lost most of their atmospheres very soon after they formed because the atmosphere drifted away).
Jupiter is made primarily of the basic materials hydrogen and helium, which is what existed near Jupiter when it was forming.
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