The planet that would float on water

Saturn is the second-largest planet in the solar system, yet also one of the lightest for its size. It is so low in density that, if it fit in an enormous bathtub full of water, it would float.

The reason is that Saturn is made up mostly of hydrogen and helium, the lightest gases there are. Its average density is lower than that of water, something that happens to no other planet in the solar system.

It is an impossible comparison in practice —there is no ocean able to hold it— but it illustrates well just how much a giant world can be, on the inside, almost all gas.

Source: NASA