Saturn is famous for its enormous rings, but one of its most surprising properties has nothing to do with what it looks like.
How can a planet float?
Saturn’s average density is lower than the density of liquid water. It is the only planet in our solar system with this distinction, according to NASA.
Density tells us how much mass is packed into a given volume. Saturn is enormous, but much of the planet consists of hydrogen and helium, so its average density is unusually low.
You may have heard that Saturn could float in a bathtub. That’s a useful way to picture the numbers, but it is not a realistic experiment: no bathtub or body of water could hold Saturn, and the planet doesn’t have a solid surface like Earth.
What makes Saturn different?
Saturn is a gas giant, with layers of gas and fluid surrounding a much denser interior. Its rings are made of countless particles of ice, dust, and rock rather than one continuous solid band.
The memorable fact is a statement about average density, not a practical invitation to set a planet afloat.
Source: NASA Science — Saturn Facts.