Least dense planet

The least dense planet in the Solar System is Saturn, which has an average density of only 687 kg/m3 (42.8 lb/cu ft). By comparison, Earth's average density is 5,513 kg/m3 (344 lb/cu ft).

The mass of Saturn is mostly made up of helium and hydrogen. Despite what might be implied by the term "gas giant", very little of the mass of Saturn is actually in the gas phase. Around 1,000 km (621 mi) below the cloud tops, the pressure creates a thick layer of helium-saturated liquid hydrogen that extends all the way to the core.

The planet's core is a ball of rock and iron surrounded by metallic hydrogen and helium ice. The density here is many times higher than Earth, but as it makes up only a small portion of the planet's radius, the overall average density remains low. Jupiter's overall composition is similar, but its proportionally larger core means that it has a higher density.