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How Do the Densities of the Terrestrial & Jovian Planets Compare?

There are currently eight recognized planets in the solar system. The inner four planets are called the terrestrial, or earth-like, planets due to a composition similar to Earth's. These planets are, in order of closeness to the sun, Mercury, Venus, Earth and Mars. The outer four planets are the Jupiter-like jovian planets. They are, in order: Jupiter, Saturn, Uranus and Neptune. Because of their differing composition, the difference in density between the two categories of planets is significant.

Density
- Density is not size or weight, although it is related to those things. In describing density, mass is the preferred term over weight because weight depends on a defined gravity. On earth, things with more mass weigh more. In space, the weight of things varies because gravity is less of a factor; but their mass stays the same. Mass is simply the amount of matter in something. Density is therefore how much material (mass) exists inside a certain volume. Planetary density can be expressed in pounds per cubic foot (lbs/cubic ft).
Terrestrials Planets
- The terrestrial planets are grouped together because of similar composition -- comprised of metals and rock. Earth's density is 344 lbs/cubic ft. It is the densest planet in the solar system. Venus is about 86 percent of the volume of Earth and has 91 percent of its mass, and thus has a density of 327 lbs/cubic ft. Mercury is much smaller -- 18 Mercurys would fit inside the Earth's volume -- but very dense due to its heavy metal composition. Its density is close to Earth's at 339 lbs/cubic ft. Mars is larger than mercury, but is the least dense of the terrestrials at 246 lbs/cubic ft.
Gas Giants
- The size of the gas giants, Jupiter and Saturn, is difficult to comprehend. Yet, their densities are much lower than that of even the least dense terrestrial planet. More than 1,300 Earths would fit inside Jupiter, and 764 Earths would fit inside Saturn. The composition of these two planets is primarily hydrogen and helium gases, with no solid surface. Due to this, Jupiter's density is only 83 lbs/cubic ft. Saturn is the least dense planet with a density of only 43 lbs/cubic ft.
Ice Giants
- Uranus and Neptune are jovian planets, but they are distinctly different from Jupiter and Saturn. They are smaller and made up primarily of a semi-liquid methane and water slush. They are often referred to as the ice giants to differentiate them from the gas giants. Uranus and Neptune are very similar in size and composition. Around 60 Earths would fit in the volume of each, but their mass is quite low. The density of Uranus is therefore only 79 lbs/cubic ft. The density of Neptune is 102 lbs/cubic ft.

Astronomy