Watch The Planets Season 1 Episode 4 Online

While a number of planets are accompanied by moons of all sizes and compositions, only the Earth boasts such a comparatively huge "companion world." What forces could have resulted in this planetary odd couple? Join the search for the answer to one of the solar system's most baffling mysteries, from Cold Warrior scientists racing to be first on the moon, to current theories and remarkable computer-generated visions of the moon's explosive creation.