Which planet has the lowest density, so low that it would float if placed in a large enough body of water?

The correct answer is B, Saturn. Saturn is a huge ball of hydrogen and helium spread over an enormous volume, and its average density is less than that of water. This makes it the least dense planet of the solar system, and the picture of Saturn floating in water is the standard way the fact is taught. Saturn is also famous for its system of bright rings and for Titan, its largest moon.

Option A, Jupiter, is the largest and heaviest planet, but it is more tightly packed than Saturn and denser than water. Option C, Neptune, is a distant ice giant and denser still. Option D, Mercury, is a small rocky planet with a large iron core and is among the densest of all. Pair the two records in your memory: Jupiter the largest, Saturn the lightest for its size.