Saturn Could Float in a Bathtub

Despite its immense size, Saturn is the least dense planet in our solar system, making it a true cosmic lightweight. This is because it's a gas giant, composed almost entirely of hydrogen and helium, the two lightest and most abundant elements in the universe. Unlike rocky planets such as Earth, Saturn is more of a vast, swirling ball of gas and liquid with a potentially small, dense core. Think of it less like a massive marble and more like a colossal ball of cosmic fluff; its mass is spread out over an enormous volume, resulting in an incredibly low average density.

This unique property is what gives rise to the famous thought experiment. Scientists determine a planet's density by first calculating its mass (by observing its gravitational pull on its moons) and then dividing that by its volume. Saturn's density works out to be significantly less than that of liquid water. This means if a body of water large enough to hold the ringed giant existed, the entire planet would bob on the surface like a cork.

This makes Saturn an outlier even among its gaseous neighbors (Review). Jupiter, for instance, is also made of hydrogen and helium, but its much greater mass compresses its interior so intensely that its overall density is higher than water's. Saturn's unique balance of size and mass allows it to remain the only planet in our solar system that could theoretically take a swim.