As far as we know, there are nine planets locked in orbit around the Sun. Only one, our own Earth, supports life. But there are countless other suns throughout countless galaxies scattered across the expanse of the universe. We still don’t know if life exists on another planet in some other galaxy.
During the past 15 years, space probes such as the Mariner and Voyager missions have given us tremendous detail about all the planets in this system.
From the Center of the Universe to the Edge of a Galaxy
All of the ancient peoples regarded the Earth as the center of the universe (a geocentric perspective), motionless under a canopy of stars that circle from east to west. The planets were perplexing to the ancients because they appeared to wander among the stars. Some of them periodically slowed down until they began moving in the opposite, western direction—retrograde motion. After a short distance, they slowed down again and resumed their eastward motion. So the ancient peoples called the planets “wanderers.”
The Solar System
Let’s look at the planets of our solar system briefly, one by one.
• The first four are known as the terrestrial planets.
• The next four are the gaseous giants.
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