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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.