— THE recent announcement by a team of astronomers that there could be as many as 40 billion habitable planets in our galaxy has further fueled the speculation, popular even among many distinguished scientists, that the universe is teeming with life. The astronomer Geoffrey W. Marcy of the University of California, Berkeley, an experienced planet hunter and co-author of the study that generated the finding, said that it “represents one great leap toward the possibility of life, including intelligent life, in the universe.”
But “possibility” is not the same as likelihood. If a planet is to be inhabited rather than merely habitable, two basic requirements must be met: the planet must first be suitable and then life must emerge on it at some stage.