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Inner versus Outer Planets
Dana Desonie, Ph.D.
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Printed: November 18, 2014
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Inner versus Outer Planets
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Inner versus Outer Planets
• Compare and contrast the inner and outer planets.
What’s better than a field trip?
All of the inner planets are orbited by man-made satellites. Jupiter and Saturn have man-made satellites too. We
can see what the planets look like from the photos they take. The satellites also carry instruments that collect a lot
of important data. This is better than taking a field trip to such hostile places. You can learn a lot while sitting your
own, very comfortable planet!
The Inner Planets
The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are the inner planets or terrestrial planets
( Figure 1.1). They are similar to Earth. All are solid, dense, and rocky. None of the inner planets has rings.
Compared to the outer planets, the inner planets are small. They have shorter orbits around the Sun and they spin
more slowly. Venus spins backward and spins the slowest of all the planets.
All of the inner planets were geologically active at one time. They are all made of cooled igneous rock with inner
iron cores. Earth has one big, round moon, while Mars has two very small, irregular moons. Mercury and Venus do
not have moons.
The Outer Planets
Jupiter, Saturn, Uranus, and Neptune are the outer planets of our solar system. These are the four planets farthest
from the Sun. The outer planets are much larger than the inner planets. Since they are mostly made of gases, they
are also called gas giants. Pictured below are the relative sizes of the outer planets and the Sun ( Figure 1.2)
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FIGURE 1.1
This composite shows the relative sizes
of the four inner planets.
From left to
right, they are Mercury, Venus, Earth, and
Mars.
FIGURE 1.2
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.
The gas giants are mostly made of hydrogen and helium. These are the same elements that make up most of the Sun.
Astronomers think that most of the nebula was hydrogen and helium. The inner planets lost these very light gases.
Their gravity was too low to keep them, and they floated away into space. The Sun and the outer planets had enough
gravity to keep the hydrogen and helium.
All of the outer planets have numerous moons. They also have planetary rings made of dust and other small
particles. Only the rings of Saturn can be easily seen from Earth.
Vocabulary
• inner planets or terrestrial planets: Four solid, dense, rocky planets that are inside the asteroid belt: Mercury,
Venus, Earth, and Mars.
• outer planets or gas giants: Four large outer planets composed of the gases, hydrogen and helium, and are
beyond the asteroid belt in our solar system: Jupiter, Saturn, Uranus, and Neptune.
• planetary rings: Rings of dust and rock encircling a planet in a thin plane.
Summary
• The four inner planets have slower orbits, slower spin, no rings, and they are made of rock and metal.
• The four outer planets have faster 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.
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Chapter 1. Inner versus Outer Planets
Explore More
Use the resources below to answer the questions that follow.
• The Planets at http://kids.nineplanets.org/intro.htm
1. Which planets make up the inner solar system?
2. Which planets make up the outer solar system?
3. What separates the inner and outer solar system?
• The Groups of Planets at http://kids.nineplanets.org/general2.htm
4. Describe the composition of each group of planets.
5. Describe the size of each each group of planets.
Review
1. Describe the characteristics of the four inner planets.
2. Describe the characteristics of the four outer planets.
3. Why are the inner and outer planets so different from each other?
References
1. User:Brian0918/Wikipedia, using images courtesy of NASA. The relative sizes of Mercury, Venus, Earth, and
Mars. Public Domain
2. User:Urhixidur/Wikipedia, using images from NSSDC. The relative sizes of the Sun, Jupiter, Saturn, Uranus,
and Neptune. Public Domain
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