- from Solar System
Rocky planets are mostly core and mantle.
They have extremely thin crusts and very thin atmospheres. Gas giants are mostly atmosphere—they have really tiny cores, considering how huge they are. Jupiter, Saturn, Uranus, and Neptune are big, swirling clouds of gas.
A cloudy atmosphere blocks our view of a gas giant, making it difficult to learn more about the planet. It’s like trying to study an egg but only being able to look at the shell. Scientists can only make educated guesses about what’s under all those clouds.