While the best way to gain an overview is to go and view one some of the ON line Solar System Tours listed on the Front Page we can begin with this summary:

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:

• Rocks
• Ices
• Gases

These properties about an object can generally be discerned by measuring their densities.

If the density is less than 1.5 grams per cc then the object is almost exclusively made of frozen volatiles such as

• Water
• Ammonia
• Methane
• Carbon Dioxide

If the density is really less than 1.0, the object must be mostly made of gas. Only one object in our solar system, Saturn, has this low of density.

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        1

What can we notice here?

By our density definition we have 2 main groups:

• Rocky Planets: Mercury Mars: These planets are called the terrestrial planets (meaning rocklike). They are also known as the inner planets.
• Icy Planets: Jupiter--Pluto: These planets are called the Jovian planets (meaning gaslike). They are also known as the outer plantes.

In addition to density, there are other important distinctions between the inner and the outer planets. These include:

• Outer planets are significantly more massive
• Outer planets have significantly larger radii

  These imply outer planets are big balls of gas, compressed by their own weight.

• Outer planets have significantly faster rotation rates they haven't been slowed down by tidal effects
• Outer planets have significantly more moons

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:

• The diversity of surface features
• Some surfaces are heavily cratered well others are not
• Heavily cratered surfaces can be found throughout the Solar System. Good examples are Mercury, the Moon, Callisto and Rhea
• Rings are common features. Saturn has the most prominent set but rings are now found around Jupiter, Uranus and Neptune
• There is considerable variation in the size of the satellites of the outer planets. Objects up to twice the size of the Moon (Calliso, Titan, Triton) can be found as well as objects of just a few 10's to hundreds of kilometers (e.g. most of Saturn's moons)
• Material in the outer part of the solar system is primarily made of ices.