Is there life elsewhere in the universe? Is it made of the same stuff as us, on an Earthlike world, or does it have different chemistry on a different kind of planet? These are fascinating questions for any space fan, but they are also frustrating, because we have so little data. At best, we can imagine what might be out there, either thought experiments firmly based in known physics or speculative fiction or some combination of the two. At the outset, Trefil and Summers’ book is an excellent introduction to the basic ideas of the study of astrobiology, explaining what experiments have shown us so far, what scientists are searching for, and the underlying physics and chemistry. The language is very accessible, always emphasizing the questions scientists are asking, how we’ve discovered answers, and how much we still don’t know. When the authors run out of facts and data—and that doesn’t take long, this is astrobiology we’re talking about—it turns speculative, considering several different kinds of exoplanetary environments where life might arise, and the reasons why (or why not) intelligent life could result. I would especially recommend this book to any kid (or adult) who enjoys writing hard science fiction, as inspiration for imagining alien life. ( )

Two scientists (physicist and astronomer -- no biologist) offer a fairly undemanding account of today's best guesses about what life forms on other worlds -- mostly exoplanets and their moons -- might be like. Each of the central chapters details a scenario for a different kind of world -- ice-crusted, iced-over ocean, open-ocean, Earth-like, tidal-locked, rogue (no parent star), and super-Earth. In a chapter on life that is not like us, the authors are hard-put to identify any plausible kind of chemical life that would not be described as "carbon-based molecules operating in water". An interesting chapter on life that is *really* not like us includes speculations about superintelligent and possibly conscious machines.