Just like the size of the planets, the density of the planets varies widely. All of the 4 inner planets – the planets closest to the Sun – are much denser than the four outer planets. That is because the inner planets, which are also called terrestrial planets, are made up of heavy metals and rocks. The outer planets are also known as gas giants and are comprised almost entirely of gases.
Mercury is the second densest planet in our Solar System. It has a density of 5.427 grams per cubic centimeter. Mars’ density is not surprising because the planet is mostly comprised of heavy metals including iron.
As an inner planet, Venus is also relatively dense compared to some of the other planets. Venus is often called Earth’s twin, and scientists believe that it has a core and a rocky crust like Earth’s. Unfortunately, scientists have not been able to test this hypothesis yet. Venus’ mass is only approximately 81% of Earth’s mass, so it is not surprising that Venus is also less dense than our own planet. Venus has a density of 5.204 grams per cubic centimeter.
Earth is the densest planet in our Solar System. It has a density of 5.51 grams per cubic centimeter. This is only the average density of the planet. The core is much denser than the oceans for example. Actually, scientists believe that the massive amounts of iron in our planet’s core is what makes it so dense.
Mars is larger than Mercury, but it has a much lower density – only 3.94 grams per cubic centimeter. Because of Mars’ low density and small mass, the planet actually has less gravity than the smallest planet Mercury.
Jupiter is the largest and most massive planet in our Solar System, but it only has a density of 1.33 grams per cubic centimeter because it is made up almost entirely of gases, such as helium and hydrogen. Even if it does have a heavy core, that core is relatively small.
Saturn is the second largest planet, but it has the lowest density of any planet in our Solar System. Saturn only has a density of .687 gram per cubic centimeter; this is actually less dense than water, which has a density of 1 gram per cubic centimeter.
Uranus is the second least dense planet in our Solar System with a density of 1.27 grams per cubic centimeter. Like Saturn and Jupiter, Uranus is an ice giant, which means that it contains “ices” like ammonia, water, and methane.
Neptune is an ice giant that has a low density of 1.638 grams per cubic centimeter. Although it has a higher density than any of the other gas giants, Neptune’s density is only about one-third of Earth’s density.
Astronomy Cast has episodes on all the planets including Mercury.