Overview of the Solar System

Solar System Composition

Constituents of the Solar System:

Planets

Satellites (moons)

Rocky interplanetary debris (asteroids)

Icy interplanetary (comets)

But note that 99.85% of the mass of the solar system is in the Sun. Below is a scaled drawing of all the diameters of the planets relative to the Sun.

Ambiguity: Yes, Pluto is in the above figure, but it is considered now a "dwarf planet" The distinction between a "Planet", "dwarf Planet" and "Moon" is made by the International Astronomical Union (IAU)

Object needs to orbit the sun
Needs to have enough mass/self-gravity to pull itself into a round shape
Needs to clear its orbit

Pluto and other large objects in the Kuiper belt do not satisfy the last requirement as can be seen in the illustration below

If a lesser body is in orbit about a bigger body, that lesser body will always be termed a "Moon".

The demotion of Pluto from a Planet to a non-planet is not a scientific issue whatsoever - its only a cultural issue.

Rather than dealing with classification names (e.g. moon or planet) its better to stick to physical definitions. Therefore, we will define solar system bodies in terms of their composition and geological properties. In these terms objects come in three categories:

Rocks
Ices
Gases

Determining the composition (rock, ice, gas) of a solar system body is done by calculating their densities.

The Concept of Density

Density is an important physical concept, therefore we will spend some extra time on this subject.

To begin with, the density of planet refers to its bulk density. So even though the structure of a planet, like Neptune below, might contain many different components of different density, they all integrate together to produce the bulk density.

The bulk density of any object is its mass divided by the volume that it occupies.

For a sphere, the volume is

But we only care about the r³ part. If you double the radius of a sphere, you increase its volume by 2³ = 8.

So for any spherical object in the solar system its density goes as

The scaling is important here as we will compute all densities with respect to the density of the earth such that 1 Earth Mass divided by 1 Earth Volume = 1 Earth Density. Or M_E/V_E = D_E, where E refers to Earth units. Alternatively, if given the radius of the planet in units of Earth radii, R_E, M_E/(R_E)³ = D_E We want to know how many more times or less times the density of an object is, compared to the Earth. This concept will definitely be tested on the midterm and the final - its fundamental to understanding the physical properties of objects.

So, for example:

An object with 2 Earth masses and 1 Earth radii would have a density twice that of the Earth: 2M_E/1V_E = 2D_E

An object with 2 earth radii and 1 earth mass would have a density 1/8 that of the earth. Or 1M_E/(2R_E)³ = D_E/8

An object with 10 earth masses and 10 earth radii would have a density of 10M_E/(10R_E)³ = 0.01D_E or 1% the density of the Earth

If you want to compute the density in proper units then you would just multiply by the density of the Earth which is 5.5 grams per cubic centimeter (g/cc). Note that the density of water is 1.0 g/cc.

The reason that density is so important is that it provides an excellent diagnostic of whether or not an object is primarily made out of rocks, ices or gases.

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 a density less than 1.0.
If the density is less than 1.5 grams and greater than 1.0 gram per cc then the object is almost exclusively made of frozen volatiles such as
- Water
- Ammonia
- Methane
- Carbon Dioxide
For densities between 1.5 and 3.0 the object is a rock-ice mixture.
If the density is greater than 3.0 grams and less than 5.0 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 large nickel-iron core in the interior of the object.

Let's look at how the properties of the planets and the two largest dwarf planets in the solar system correlate with distance from the Sun:


  Planet       Mass      Radius    Density     Rotation    #Moons
             (earth=1)  (earth=1)   g/cc         days

  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+
  
  Dwarf 
  Planets
  Pluto        .002       .18        2.03        6.4d        1
  Eris         .0028      .183       2.52        ??          1

What can we notice here?

By our density definition we have 2 main groups of objects:

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

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 and other Kuiper belt objects have larger densities than the large gas giants. Effectively they are large rock-ice objects - comets. 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.