宇宙人はどこにいるのでしょう？

A classic of astrobiology, which defends the hypothesis that complex life might be rare in the cosmos. Ward and Brownlee cover a vast amount of ground — solar system dynamics, plate tectonics, abiogenesis — but do so with clarity and style.

A subtle and beautifully argued account of the various factors that influence planetary habitability and permit the evolution of advanced life forms. A key focus is the relatively stable climatic conditions that Earth has enjoyed over the past few billion years. The luck involved in finding oneself on a planet possessing such climate stability might strain one's credulity — except that there could be ten billion trillion planets in the universe, so it's going to happen somewhere. And it's only on those lucky planets that life can develop sufficiently to be in a position to be able to ponder these questions.

Look at one of your body's cells through a microscope. Then look at, say, a cell from a mushroom. You won't be able to tell them apart. But, as Nick Lane points out, you don't live much like a mushroom — so why are your cells so similar? Lane argues the answer has a lot to do with mitochondria — the power plants that work inside all complex, eukaryotic cells to provide energy. But the development of the eukaryotic cell might well have been a freak event. If it was, we might live in a galaxy in which simple life is common but complex life is rare.

In contrast to Nick Lane's view, the astrobiologists Schulze-Makuch and William Bains argue that any world that hosts life will almost certainly go on to develop a complex biosphere. They identify two caveats. First, perhaps abiogenesis — the origin of life — is a highly improbable event. If that's so, we live in an empty universe. Second, we don't know whether complex life is likely to go on to develop technological intelligence. If human-level, technological intelligence is indeed rare, then don't expect any inhabitants of the cosmic zoo to visit us any time soon.

If we don't look how can we know whether we're alone? In this evocative account. Billings writes beautifully not only of the search for exoplanets and extraterrestrial intelligence, but also of what drives people to continue searching.