Jupiter or Saturn, primarily

GAS

Think of what makes these planets much lighter than Earth despite their massive size—it’s the same state as helium in a balloon.

This clue hints at the main composition of Jupiter and Saturn, which are both gas giants—planets made mostly of hydrogen and helium. The word "primarily" signals that the answer focuses on their dominant feature, "gas," rather than their names. It’s a straightforward definition clue with no hidden wordplay, just a nod to basic astronomy.