Saturn would float in a swimming pool

Saturn's average density is about 0.7 grams per cubic centimetre — lower than water's. On paper, if an ocean big enough existed, the ringed planet would bob on the surface like a giant cork.

The reason is its recipe: Saturn is almost entirely hydrogen and helium, the two lightest elements in the universe, puffed up into an enormous ball. At the other extreme sits Earth, the densest planet in the solar system, with its iron heart.

The experiment, mind you, is doubly impossible: there is no bathtub more than 120,000 kilometres wide, and besides, Saturn isn't solid — dunk it in water and it would simply come apart. But the fact stands: the lord of the rings, for all its size, weighs far less than it looks.