This book is an extremely useful addition to astrobiology lore. By focusing on the four prerequisites for life (energy, chemistry, solvent and habitat), the authors embark on a carefully constructed examination of the possibilities of the forms that life might take, ruthlessly slaying Earth-centrism that is so dominant in astrobiology. Each chapter is concluded with a useful summary and an extensive reference list. The first introductory chapter defines the book as a whole, presenting a short summary of their approach and rationale. The second chapter, on the definition of life, goes beyond the traditional notions of life, and re-examines its properties and overturns conventional thinking. In the process, the authors are armed with cases – many new to me – to support their approach. Of particular note is their emphasis on the primacy of biomembranes to maintain thermodynamic disequilibrium. They then outline the implications of their definition of life for the origin of life adding biomembrane formation to the metabolism/replication-first debate. The third chapter, on lessons from the history of life, attempts to generalize from the specifics of our own biosphere through a number of observations, for example life tends to stay small and simple; complexity inevitably increases but as the exception rather than the rule, etc. These first three chapters set the scene with the following chapters delving into their analysis in more detail. The fourth chapter, on energy sources and life, starts off with the known redox reactions and then examines a number of different possibilities for powering life, from thermal gradients to osmotic gradients to magnetic fields and others, in a remarkably succinct assessment of what may be possible. In the fifth chapter, on building blocks of life, the authors initially consider carbon compounds, and then expand to alternatives such as silicon and variations thereof, and other possibilities such as boron. The sixth chapter, on life and the need for a solvent, begins with water and then expands to other polar solvents such as ammonia, hydrazine, acids, etc. Even non-polar solvents are considered. The seventh chapter, on habitats for life, considers surface, sub-surface, atmospheric and space environments as potential habitats. The eighth chapter, on ideas of exotic forms of life, expands the consideration of life to spin configurations, the Black Cloud (from Fred Hoyle), the neutron star (from Robert Forward) and brown dwarf habitats. The ninth chapter, on signatures of life and the question of detection, examines the notion of biosignatures, geosignatures and geomindicators, concluding that instruments should be attempting to detect energy gradients as one of several different approaches. This last chapter was the left-field chapter in that it did not seem to conclude what I expected – that the variations of life forms might be so vast and different that we might not recognize them. More importantly, we find what we seek, and if we ask the wrong questions we will not get the right answers. However, that said, the authors have highlighted that we need to be very careful about how we design instruments for life detection if the shades of Viking are not to be repeated.

This is very much a book for the specialist and it is a major contribution to the science of astrobiology. It is rich with information and its thoroughness is delightful. It is mind-expanding and I know of no other book that re-assesses the fundamentals of astrobiology in such a way. This book is a tacit lesson in open-mindedness tempered with thorough scientific analysis. This is a very important book for all professional astrobiologists.