Answer:
The inner four planets have densities of about 5gm/cm3 , while the outer planets have densities of about 1gm/cm3 . Pluto doesn't fit with the other; it has a density of about 2gm/cm3 . To get a feel for what these numbers mean, you have to be able to realize that water has a density of 1gm/cm3 .
although Saturn is mainly composed of the lightest two gases known, hydrogen and helium. It is the only planet in our solar system whose density is less than water.