20.6 Inner vs. Outer Planets

[Figure 1]

"The Sun, with all those planets revolving around it and dependent on it...

"...can still ripen a bunch of grapes as if it had nothing else in the universe to do." — Galileo Galilei

The Inner Planets

The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Figure below shows the relative sizes of these four inner planets.

[Figure 2]

Unlike the outer planets, which have many satellites, Mercury and Venus do not have moons, Earth has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron cores, and all have been geologically active, at least early in their history. None of the inner planets has rings.

The Outer Planets

The four planets farthest from the Sun are the outer planets. Figure below shows the relative sizes of the outer planets and the Sun. These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants.

[Figure 3]

The gas giants are made up primarily of hydrogen and helium, the same elements that make up most of the Sun. Astronomers think that hydrogen and helium gases comprised much of the solar system when it first formed. Since the inner planets didn’t have enough mass to hold on to these light gases, their hydrogen and helium floated away into space. The Sun and the massive outer planets had enough gravity to keep hydrogen and helium from drifting away.

All of the outer planets have numerous moons. They all also have planetary rings, composed of dust and other small particles that encircle the planet in a thin plane.

Summary

• The four inner planets have shorter orbits, slower spin, no rings, and they are made of rock and metal.
• The four outer planets have longer orbits and spins, a composition of gases and liquids, numerous moons, and rings.
• The outer planets are made of hydrogen and helium, so they are called gas giants.

Review

1. What are the four inner planets? What are the four outer planets?
2. What is the difference in composition between the inner and outer planets? What accounts for the difference?
3. Why do the outer planets have more moons? Why do they have rings?

Explore More

Use this resource to answer the questions that follow.

1. Which are the inner planets?
2. Which are the outer planets?
3. Where are the two groups?
4. What are the sizes? What are the masses?
5. What is the composition of the inner planets and how does that affect density
6. What sort of bodies do the inner planets have? What sort of atmosphere?
7. What sort of bodies do the outer planets have? What sort of atmosphere?
8. What is the rotation of the inner versus the outer planets?
9. What is the orbital speed of the inner versus the outer planets?
10. Which planets have the most moons, inner or outer? Why is this?
11. Which of the two planet groups have rings?