A: What is the Solar System?

Among otherwise well-educated people, it is common to hear the terms Solar System, Galaxy, and Universe interchanged.  For instance, you might hear "Jupiter is the biggest planet in the Galaxy," or the question, "How many stars are there in our solar system?"  These may seem silly to those who know a bit about the subject, but even knowledgeable people are not really sure what constitutes the solar system, what objects are part of it and what objects are not--in short, what constitutes the boundary between our solar system and the rest of the galaxy.  This lecture will hopefully give you a feeling for what is part of the solar system and what is not, and will also give you some idea of the size scale of the solar system.

As the name implies, the solar system has something to do with the Sun, or Sol.  (Incidentally, the words Sun, Moon, Earth, Mars, etc., should be capitalized since they are proper names.)  The Sun dominates and controls the solar system, mainly by its gravitational influence (keeping the planets in their orbits), but of course its light, heat, and other forms of energy are important also.  We will learn of another important way that the Sun dominates its surroundings--through its magnetic field.

Most people would agree that the solar system is made up of

• the Sun
• the planets

• the asteroid belt.

• other asteroids
• comets
• meteroids
• dust
• even elementary particles (protons and electrons of the solar wind).

B: The Sun and Planets

Let us list the major components of the solar system, the Sun and planets, in order of their distance from the Sun.

• Sun
• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn      | S
• Uranus     | U -- Spells SUN  (helps to remember the order)
• Neptune   | N
• Pluto

1. Characteristics of the Planets

When we look at the solar system as a whole, we see a number of patterns that give clues to how the solar system might have been formed. Here are a few things we might notice:

• The planets all revolve (orbit) around the Sun in the same direction (this is counter-clockwise [CCW] as seen looking down from above the north pole of the Earth).
• The inner four planets--Mercury, Venus, Earth, Mars--are small, rocky bodies. These are called the terrestrial planets.
• The next four planets--Jupiter, Saturn, Uranus, Neptune--are large gas giants, made mostly of hydrogen and helium, but with methane, ammonia, and other gases. These are called the jovian planets.
• The planets all orbit close to the same orbital plane as Earth (the ecliptic). Only Mercury and Pluto are very far off, with Mercury's orbit tilted at 7 degrees, and Pluto's at 17 degrees from the ecliptic.
• The separation of the planet orbits increases as you go out from the Sun, with the jovian planets almost doubling in distance at each step.

There are other patterns that we will talk about next time, but let's first get an idea of the size of the solar system.

2. Scaling the Solar System

We want to get a feeling for the immense size of the solar system.  To do that, we will look at distances and sizes of the planets, and try to put them into a scale that we can understand.

Let's make our own table:
 

Object	Orbital Radius D (x 106 km)	Orbital Radius (AU)	Radius  R (km)	Radius  RE	D/Ro	R/Ro
Sun	---		696,000		---	1.0
Mercury	57.9	0.39	2,439	0.38	83	0.0035
Venus	108.2	0.72	6,052	0.95	155	0.0087
Earth  --Moon	149.6	1.0	6,387 1,738	1.0 0.27	215	0.0092 0.0025
Mars	228.0	1.5	3,393	0.53	328	0.0049
Jupiter	779.3	5.2	71,398	11.2	1118	0.1026
Saturn	1427	9.5	60,000	9.5	2050	0.0862
Uranus	2871	19.2	25,559	4.0	4125	0.0367
Neptune	4497	30.1	24,800	3.9	6461	0.0356
Pluto	5913	39.5	1,140	0.2	8496	0.0016

 

Now, looking at these numbers, we get a really good notion of the vast scale of the solar system, right? ...No!  It is not possible to comprehend the scale by looking at numbers alone.

3. The Thousand-Yard Model

Let's build a scale model of the solar system, right here in this room.  First we need to choose a convenient scale.  Note that the whole purpose of this exercise is to allow you to get a "gut-feeling" for the size and scale of the solar system, and since for better or worse, we use inches, yards, and miles in the U.S., it is better to use those units here.

Since the Sun is 696,000 km in radius, its diameter in miles is about 860,000 miles.  Let us choose 1" = 100,000 miles, so that our Sun will be about 8.6 inches in diameter.  This same scale gives the radius of the Sun in yards as 4.3 inches/36 inches/yd = 0.12 yd.  So let's make a new table using this scale:
 

Object	Distance D (yards)	Diameter R (inches)	Model Object
Sun	---	8.6	Melon
Mercury	10	0.03	Mustard seed
Venus	19	0.08	Peppercorn
Earth  --Moon	26	0.08 0.02	Peppercorn
Mars	39	0.04	Mustard seed
Jupiter	134	0.88	Walnut
Saturn	246	0.74	Acorn
Uranus	495	0.32	Peanut
Neptune	775	0.31	Peanut
Pluto	1020	0.01	Mustard seed

We will now take volunteers to "be" the planets, hold the object, and pace off the distance.  Since each step is about 1 yd, we can pace off the distance easily.  The rest of this lecture takes place outside.  See The Thousand-Yard Model for a description and more information.

As a result of this lecture, you should have a much better idea of the scale and size of the solar system.  For the next few weeks we will be discussion the motions of planets and other solar system bodies, both as seen from our vantage point on the Earth and as would be seen from a fixed point in space.  When we discuss planetary orbits, we will get into some rather heavy mathematics and physics, but keep in mind that we are talking about something really very simple--the motions of these little "seeds and nuts" in a vast volume of empty space, under the influence of a far-reaching, but rather weak central force, the force of gravity.