First, the distinction between a "Planet" and a "Moon" is fairly hazy as there are Moons which are bigger than some Planets. Hence the definition is strictly one of orbital parentage.
We wish to define solar system bodies in terms of their composition and geological properties. In these terms objects come in three categories:
If the density is less than 1.5 grams per cc then the object is almost exclusively made of frozen volatiles such as
If the density is greater than 3.0 grams per cc then the object is almost exclusively made of rocks. If the density exceeds about 5.0 grams per cc then there must be a nickel-iron core in the interior of the object. For densities between 1.5 and 3.0 the object is a rocky-ice mixture.
Let's look at how the properties of the planets in the solar system correlate with distance from the Sun:
Planet Mass Radius Density Rotation #Moons Mercury .05 .39 5.42 58d 0 Venus .81 .95 5.25 243d 0 Earth 1 1 5.54 1d 1 Mars .11 .53 3.94 1.02d 2 Jupiter 318 11 1.31 9.8h 15+ Saturn 95 9.5 0.70 10.2h 19+ Uranus 14.5 4.0 1.29 17.9h 9+ Neptune 17 3.9 1.64 19.1h 6+ Pluto .002 .18 2.03 6.4d 1What can we notice here?
By our density definition we have 2 main groups:
In addition to density, there are other important distinctions between the inner and the outer planets. These include:
These imply outer planets are big balls of gas, compressed by their own weight.
Note: Pluto really doesn't belong, at all, in the same category as the outer planets as it shares none of their properties. Its an extremely small object that just happens to be in orbit about the Sun and hence is called a planet. In reality, Pluto is basically a large comet. The orbit of Pluto is also much more eccentric and inclined to the orbital plane than any of the other planets.
All of these distinctions are clues to how the overall solar system formed. To gain further insight, let's take a tour of The Nine Planets
Things to notice on the tour of the Planets plus their Satellites: