TEMPE, Ariz. — 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.

What can be said about the chances of life starting up on a habitable planet? Darwin gave us a powerful explanation of how life on Earth evolved over billions of years, but he would not be drawn out on the question of how life got going in the first place. “One might as well speculate about the origin of matter,” he quipped. In spite of intensive research, scientists are still very much in the dark about the mechanism that transformed a nonliving chemical soup into a living cell. But without knowing the process that produced life, the odds of its happening can’t be estimated.