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CASE STUDY
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.
Figure 1: The Solar System
Mercury
Named for the wing-footed messenger of the Roman gods, Mercury races around its orbit at a
dizzying speed of 30 miles (48 kilometers) per second, making the Mercurial year only 88
Earth days long. In contrast, one rotation around its axis—or a single day—takes almost 59
Earth days.
Daytime temperatures rise to about 800 degrees Fahrenheit (341.33 degrees Celsius), which is
not surprising given its close proximity to the Sun. That proximity makes it difficult to study
Mercury from the earth because of the interference of the Sun’s brilliance.
Mariner 10 gave us a wealth of information about Mercury when it approached the planet in
1974 and 1975. We learned that Mercury has an extremely weak magnetic field, which could
indicate a hot metallic core, such as molten iron. Geologists think Mercury may be the most
iron-rich planet in the solar system. Mercury’s crust seems to be silicate, like that of Earth.