- from Solar System
Rocky planets are mostly made of a core and a mantle.
They have very thin crusts. Their atmospheres are very thin, too. Gas giants are mostly atmosphere. They have very tiny cores compared to their size. Jupiter, Saturn, Uranus, and Neptune are big, swirling clouds of gas.
A cloudy atmosphere makes it hard to see all of a gas giant. It’s like looking at an egg but seeing only the shell. Scientists can only guess what’s under all those clouds.