Question

Why are "Outer planets" also called gas giants?​

Answer 1

"Outer" planets are also called gas giants because they are large planets that have deep, massive atmospheres made mostly of gas.

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Answer 2

Outer planets are also called gas giants because they are much larger than Earth and the other inner planets, and because they are made primarily of gases and liquids rather than solid matter, the outer planets are also called gas giants.