Which planet is less dense than water?

Saturn is the planet less dense than water. Its average density is about 0.69 grams per cubic centimetre, compared with roughly 1.00 for liquid water, making Saturn the only planet in the Solar System that would theoretically float in a vast enough bathtub.

This unusually low density comes from Saturn’s composition and enormous size. The planet is a gas giant made primarily of hydrogen and helium, and its volume is about 764 times Earth’s while its mass is only about 95 times greater. Gravity compresses the deep interior into liquid and metallic forms, but the overall average remains exceptionally low.

Saturn is often confused with Jupiter because both are hydrogen-rich gas giants, but Jupiter is denser, with an average density greater than water. Saturn’s rings do not make it light; they are mostly water-ice particles and contribute very little to the planet’s total mass. The “floating planet” description is a comparison of average density, not a claim that Saturn could float in an ordinary ocean.