Points to Consider
- We are planning to send humans to Mars sometime in the next few decades. What do
you think it would be like to be on Mars? Why do you think we are going to Mars
instead of Mercury or Venus? - Why do you think the four inner planets are also called terrestrial planets? What
might a planet be like if it weren’t a terrestrial planet?
25.3 Outer Planets
Lesson Objectives
- Describe key features of the outer planets and their moons.
- Compare the outer planets to each other and to Earth.
Introduction
The four planets farthest from the sun—Jupiter, Saturn, Uranus, and Neptune—are called
theouter planetsof our solar system. Figure25.19shows the relative sizes of the outer
planets and the Sun. Because they are much larger than Earth and the other inner planets,
and because they are made primarily of gases and liquids rather than solid matter, the outer
planets are also calledgas giants.
Figure 25.19: This image shows the four outer planets and the Sun, with sizes to scale. From
left to right, the outer planets are Jupiter, Saturn, Uranus, and Neptune. ( 18 )
The gas giants are made up primarily of hydrogen and helium, the same elements that make
up most of the Sun. Astronomers believe that hydrogen and helium gases were found in
large amounts throughout the solar system when it first formed. However, the inner planets