This book is mainly about possibilities rather than actualities, as we have no hard evidence for the existence of extraterrestrial life. We do, however, have plenty of scientific data which indicate that some forms of life similar to that here on Earth could exist on other planets and satellites in the solar system. Life could also be common throughout the universe. As Jeffrey Bennett reminds us, astronomers in the first half of the twentieth century thought that planetary systems around stars were rare because they favoured a theory that the planets of our solar system were formed as the result of a very rare near-collision between the sun and another star billions of years ago.
It is now known that planetary systems are common, although at present it is possible to detect only the larger planets. Expected improvements in detection techniques should soon result in finding extrasolar planets about the same size as Earth or smaller. This has encouraged much scientific speculation about life, intelligent or otherwise, throughout the universe.
The author states: "In terms of possibilities for life in the universe the first thing to understand is that the universe is big, really BIG." (He is obviously referring here to the observable universe. Some cosmologists believe that the universe is infinite.) To give us some idea of the numbers involved, we are told: "The total number of stars in the sky is roughly the same as the total number of grains of sand on all Earth's beaches put together."
A chapter is devoted to discussing the nature of life and we are informed that the greatest biomass and greatest variety of life on Earth consists of microorganisms rather than animals and plants. It is known that some of these can thrive under extreme conditions, so that they could undoubtedly live on other planets or satellites in our solar system. The only other planet which might support life is Mars, where microorganisms could live deep underground where liquid water could exist.
Life could also exist on a few of the larger satellites, such as Jupiter's Europa, which is covered by an ocean of water, frozen on the surface, but no doubt liquid below the ice, the necessary heat being produced by gravitational stress.
Of course, what most readers of this book will be interested in is not just life, but intelligent life, but despite the use of the term "UFOs" in the title, there is little in-depth discussion of SETI (search for extraterrestrial intelligence). Bennett confines his remarks to the search for radio signals sent by civilisations.
He does mention the possibility that there is a highly advanced galactic civilisation but that we do not yet have the technology to detect its activities, but he doesn't mention the fact that for many years some scientists have advocated looking for signs of it somewhat closer than stars many light years away by trying to detect devices such as Bracewell probes (automated spacecraft sent to survey planetary systems and possibly making contact with any civilisations it might discover) and von Neumann probes (similar devices but having the ability to construct replicas of themselves from local materials).
The author has provided a useful introduction to the possibilities of ET life for the general reader, as no great scientific knowledge is assumed. In places, though, the writing seems a bit too informal, giving the impression that it is based on notes for lectures delivered to not-very-bright students. - John Harney.