The least dense planet is Saturn, with a density of 0.687 g/cm 3 . Saturn is the only planet in the Solar System that would float on water (given a large enough ocean). If the star is large enough, gr...The least dense planet is Saturn, with a density of 0.687 g/cm 3 . Saturn is the only planet in the Solar System that would float on water (given a large enough ocean). If the star is large enough, gravity can be strong enough to push the electrons of the atoms into the nucleus, forming a “neutron star”, which is essentially a giant ball of neutrons.
