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The giant impact theory explains that billions of years ago, an object the size of Mars collided with the Earth. The resulting debris, which was hurled into orbit, consolidated to create the Moon. New observations from NASA's Spitzer Space Telescope suggest that moons like Earth's—that formed out of catastrophic collisions—are uncommon in the universe, arising at most in only 5 to 10 percent of planetary systems. The scientists calculated this probability from their observations of about 400 stars, all about 30 million years old.