Saturn is less dense than water, but it wouldn't float

Saturn is the only planet in the Solar System less dense than water: its mean density is about 0.69 grams per cubic centimetre, against 1.0 for water. That is why NASA says it could float in a bathtub, if a bathtub that big existed.

The reason is what it is made of. Saturn is built mainly from hydrogen and helium, the two lightest elements, and ESA puts hydrogen at 93% or more of its atmosphere. Spread across a planet about nine times as wide as Earth, with an equatorial diameter of roughly 120,500 km, that light material makes a huge volume with little mass for its size.

But the 0.69 figure is an average, and Saturn is far from uniform inside. NASA describes a dense central core of metals such as iron and nickel, wrapped in rocky material, beneath a layer of liquid metallic hydrogen and then ordinary liquid hydrogen. Royal Museums Greenwich estimates the rocky core at about the size of Earth, with the metallic hydrogen shell around it some 30,000 km deep. Those inner layers are much denser than water. It is the vast, light outer envelope that pulls the planet’s average below 1.

“Gas giant” is a little misleading too. Saturn has no true surface, and NASA notes that it is mostly swirling gases near the top and liquids deeper down, where pressure squeezes hydrogen until it behaves like a liquid metal.

So the bathtub is a thought experiment, not a forecast. Saturn has no solid shell to bob like a beach ball, and its core is far heavier than water, so only the planet’s average, not the planet itself, passes the floating test. The figure behind the image still holds, though: Saturn is the least dense of all the planets.