Thousands of years ago, people looked at the sky in wonder, fascinated by the motions of a few wandering “stars.” Nobody understood where these wandering objectsnow named Mercury, Venus, Mars, Jupiter, and Saturncame from, why they moved, or what drove their motions through the sky. Today, people know these objects are planets, but the quest to reach this understanding took thousands of years, and the consequences were profound. Famous scientists Johannes Kepler, Edmund Halley, Isaac Newton, and others discovered the laws of gravity and planetary motion, using these laws to explain the workings of the solar system. Their findings allowed the human race to find its way from planet to planet with unmanned probes and eventually allowed people to reach the moon. In Planetary Motion, learn how scientists have found new planets outside the solar system, and continue their search for planets like Earth.