Chapter 6: Other Worlds: An Introduction to the Solar System

In the last 3 decades we have learned a tremendous amount about out solar system. We have now sent spacecraft to the Moon, to every planet save Pluto, to some of the larger and more interesting moons, to comets, and even to an asteroid.

In this chapter we get an overview of the solar system, and discuss some of the ways in which we classify and study the objects in it. We finish with a discussion of the present theory about how the solar system formed.

6.1 Overview of Our Planetary System

An Inventory

A star is an object that produces its own energy, usually seen in the form of light. Our star is the Sun. We will discuss the Sun in detail later in the course. The chapters on the solar system are concerned with the other objects: planets, moons, rings, asteroids, and comets.

When we consider the objects of the solar system in terms of their mass, we find that the Sun accounts for almost all of it. According to table 6.1, the Sun accounts for 99.80% of all the mass of the solar system, that is all but 0.2%! Of the remainder, Jupiter (the largest planet) accounts for half, 0.10%. Stated another way, Jupiter is about 1/1000 as massive as the Sun.

The masses of the Sun, Earth, Jupiter, and other objects in the solar system are measured by observing the effect of their gravity on other objects. While this has been done since the time of Newton, modern measurements can be made even more precise by directly measuring a planet's gravity on the trajectory of nearby spacecraft.

The solar system has nine planets, in order of their (average) distance from the Sun:

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Jupiter
  6. Saturn
  7. Uranus
  8. Neptune
  9. Pluto.
All of the planets circle the Sun in the same direction. Their orbits are all (approximately) in the same plane.

All of the planets rotate (spin) about an axis. In most cases, the direction of the spin is the same as the direction of their revolution about the Sun. The exception is Venus, which spins "backwards". And Uranus and Pluto spin "on their sides", with their axes pointing in the plane of the solar system.

The planets are divided into two categories, based on broad similarities:

  1. The four innermost planets (Mercury, Venus, Earth, and Mars) are called the terrestrial planets. The terrestrial planets are small, composed of rock and metal, and have solid surfaces.
  2. The next four planets (Jupiter, Saturn, Uranus, and Neptune) are called the jovian or giant planets. The giant planets are larger, composed primarily of liquids and gases, and don't have solid surfaces.
  3. Pluto is in neither of these categories. There is some debate over whether Pluto is a planet or should be classified with objects in the Oort cloud.

Smaller Members of the Solar System

All the planets except Mercury and Venus have moons (small m).

The Giant planets have rings (all, not just Saturn) composed of small to medium sized chunks of material.

Asteriods are rocky metallic objects. Most orbit between Mars and Jupiter, in the asteroid belt.

Comets are chunks of ice that orbit, for the most part, beyond the orbit of Pluto. They are not just water ice, but also frozen gases such as carbon dioxide and carbon monoxide.

A Scale Model of the Solar System

Demonstrate a model of the solar system. Let 2.8×108m = 1 cm.
Mars0.24mm810cm = 8.1m
Jupiter5.1mm2800cm = 28m
Saturn4.3mm5100cm = 51m
Uranus1.8mm10000cm = 100m
Neptune1.8mm16000cm = 160m
Pluto0.08mm21000cm = 210m

6.2 Composition and Structure of Planets

We can group the nine planets by composition into the giant planets (Jupiter, Saturn, Uranus, and Neptune), the terrestrial planets (Mercury, Venus, Earth, and Mars), and Pluto.

The Giant Planets

The giant planets have a core of metal, rock, and ice, surrounded by large amounts of hydrogen and helium. While hydrogen and helium are gases in Earth's atmosphere, the larger gravity of the giant planets compresses them into liquid form. These planets have essentially no solid surface to stand on. One would simply continue through layers of liquid at increasing pressures, eventually reaching pressures high enough to crush any object.

The Terrestrial Planets

The terrestrial planets are quite different in their composition. They are composed primarily of rocks and metals. The principal elements of these rocks and metals are silicon, oxygen, nickel, and iron. These planets have differentiated, meaning that the interior of the planet is or was molten for a sufficiently long time to allow the heavier elements to settle to the core.

Moons, Asteroids, and Comets

Temperatures: Going to Extremes

The general trend is that the further a planet is from the sun the cooler it is. Venus, and

Geological Activity

There is good evidence that all the planets have seen considerable bombardment by projectiles from space. The terrestrial planets and some moons show evidence of geological activity (volcanos and seismic quakes) produced by internal heat.

6.3 Dating Planetary Surfaces

How do we know the age of a planet's surface (supposing it has a surface we can view)? There are generally two techniques used: counting craters and radioactive dating.

Counting the Craters

Crater counting relies on knowing the rate of impacts during the past. The number of craters on the surface tells us how long since the surface experienced a major change from volcanos, surface movement, or erosion by wind or water. From this, astronomers can date the exposed surface (not the same as dating the planet).

Radioactive Rocks

Radioactive dating of rocks relies on the natural abundances of certain radioactive isotopes and their decay properties. This technique was used to determine that the Earth and Moon are about the same age, having formed some 4.5 billion years ago.

Radioactive dating relies on the fact that, although we don't now when any one atom of a radioactive isotope will decay, we know that, for a large number, on average, half will decay in one half-life. (Figure 6-11)

6.4 Origin of the Solar System

Where did the solar system come from? How did it form?

Looking for Patterns

We can hope to find some clues from regular patterns that appear in the solar system. For example, we've found that all the planets revolve around the Sun in the same direction, and their orbits lie in nearly the same plane. The Sun spins on its axis in the same sense as well. This leads to the hypothesis that the Sun and planets formed at the same time from a rotating cloud of gas and dust called the solar nebula.

The Evidence from Far Away

Building Planets

© Robert Harr 2001