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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 galaxiesscattered 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 havegiven 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 theuniverse (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 ofthem 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 theancient 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 arethe 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) persecond, 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 degreesFahrenheit (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’sbrilliance.
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...