Saturn is the only planet light enough to float on water
Saturn is so puffed up with hydrogen and helium that its average density is lower than water's, so in theory it would bob in a big enough ocean.
Density is mass divided by volume, and Saturn is a study in being enormous yet remarkably empty. Its average density is roughly 0.7 grams per cubic centimetre, while liquid water sits at 1.0. That makes Saturn the only planet in the Solar System whose overall density is less than water’s, which is why NASA cheerfully notes it would float if you could find a bathtub big enough to hold it.
The reason is its recipe. Saturn is built almost entirely from hydrogen and helium, the two lightest elements in the universe, spread across a body about nine times Earth’s diameter. Lots of volume, not much heft.
A planet you could, on paper, set adrift.
The bathtub image is a thought experiment rather than a forecast, of course. Saturn has no solid surface to keep it intact, and its dense core would happily sink while the gaseous outer layers spread across the water as a new atmosphere. Still, the underlying number is real and genuinely strange.
Sources & references
2 referencesWell-established. Corroborated by 2 independent sources.
