EXTRATERRESTRIAL LIFE, ALSO CALLED ALIEN LIFE, IS LIFE THAT DOESN'T ORIGINATE FROM EARTH. THESE LIFEFORMS MAY RANGE FROM SINGLE CELLED ORGANISMS TO BEINGS WITH CIVILIZATIONS FAR MORE ADVANCED THAN HUMANITY. THE SCIENCE OF EXTRATERRESTRIAL LIFE IS CALLED EXOBIOLOGY. the universe is home to a lot of stars. And as research over the past decade has shown, perhaps at least 50 percent of those stars harbor planets. Shostak estimates there are 1 trillion planets in the Milky Way alone. "Surely some of them have undergone what Earth has undergone and developed life, and eventually what we call sentient life," he says. The argument, he notes, is simply one of probability. "If we are the only intelligent beings in the galaxy, or for that matter in the universe, then we are truly a miracle,"