Continuing discoveries of ever more extreme environments in which carbon-based, water-reliant life have been found on Earth provide an ever expanding volume of the Habitable Zone (HZ), the distance from a star in which liquid water can exist. The book is organized into an introduction and five sections, each of which becomes more speculative than the last. The first section, on life as we know it, and as we may come to know it, wrestles with the fundamental problem of defining what is “life”. The authors refuse to exclude novel chemical building blocks such as silanes. But any coherent discussion has to narrow the options to make headway, and so ‘Carbon Chauvinism’ (originally described by Carl Sagan 1973 as a limiting idea) eventually wins through.
‘Water Orthodoxy’ is explained and inevitably accepted, and so later chapters on the prospects for life on the terrestrial planets, and habitable sites in the outer solar system, draw on our greater understanding of the location of liquid water. The authors use the variation over time of the HZ - to areas well outside the traditional Earth-centric orbit – and biotic chemistry to examine the possibilities of early life on Venus (3.5Ga ago) and Mars, within the satellites of the gas giants, and within comets and other trans-Neptunian objects.
The last two chapters would have been inconceivable just two decades ago. The discovery by NASA’s Kepler mission of several thousand candidate planets, and several dozen confirmed extra-solar planets, and the subsequent measurement of the atmospheric composition of such bodies, has given Astrobiologists much to consider. The final chapter reads like a science fiction novel, discussing as it does large human habitats in space, terraforming the terrestrial planets and the evolution of our Habitable Zone.
The production of the book is excellent, with black and white illustrations at relevant points in the text, which are then reproduced in full colour in a central section. This neat little volume provides an enjoyable and state of the art review of the search for life outside the traditional HZ. The next generation of space telescopes, and missions such as NASA’s New Horizons and the ESA’s JUICE to the icy moons of Jupiter, will no doubt render some of the content obsolete, but until then it will be of interest to graduate students of planetary geology and philosophers on the meaning of life.
Reviewed by Lewis McCaffrey
LIFE BEYOND EARTH by Athena Coustenis and Thérèse Encrenaz. Cambridge University Press, ISBN 9781107026179, pp 287, List Price: £29.99