Our galaxy is made up of some four hundred billion stars and at least a hundred billion planets. How many are like Earth, with an atmosphere, flowing water, complex geology, and abundant life? Astronomers are seeking answers in countless photons racing past the Earth and in bold new theories about how planets form and evolve. What they are finding has only sharpened one of the oldest debates in science: Is biology a powerful and inevitable byproduct of cosmic evolution? Or is it rare, in a galaxy shaped by violence and mayhem?