The following picture will help you to better understand the densities of the planets. Below we have pictures of common objects and their densities. The least dense object, cork, is on the left and the most dense object, iron, is on the right.  Study the pictures and think about why they have those densities. You will be answering questions about the locations of the planets relative to the Sun and where to place the planets on this density scale. To help you do that you can use the following Planet Link. Ignore Pluto!

What did you find? Where are the denser planets? Where are the less dense ones? This must tell us something about how the Solar System formed. The Sun and planets formed out of a rotating disk of gas and a relatively small amount of dust. The planets built up by the collision and sticking of solid materials in the disk. See the artist’s conception.

(Bill Hartmann)

As the Sun was forming at the center and heating up, the inner part of the disk was quite hot, while the outer part was cool.  As a result, the only solid material in the inner part available to build planets from was the grains of dust. Everything else was gas.  The grains collided and stuck, eventually building rocks and finally the inner planets.  They weren’t big enough to trap much of an atmosphere, and so the density of these planets reflects the rock and metals that comprise them.  These are the Terrestrial Planets.

In the outer parts, however, some of that gas could turn into ice, and so there was much more solid material.    That led to the formation of much larger solid planet cores which were able to trap huge amounts of gas around them because of their gravity.  Thus, most of these planets are low density gas envelopes.   These are the Jovian Planets.