Rare Earth proposes that Complex life is rare in the Universe. Paradoxically it first points out how abundant simple life is on Earth, in deep ocean vents, several km below the Earths surface, geysers: places that a few decades ago we would have considered completely inhospitable turn out to host life. But, they argue, complex animal and plant life (eucaryotic life) takes special conditions that they argue are probably unique.

This book is an excellent starting location for someone learning about the state of the art in astrobiology. It summarises well the myriad sciences involved in why life exists on Earth, from astrophysics (is our place in the Galaxy special), planetary development (why only Earth appears to have life in our Solar system), through aspects of geology and biology that we begin to understand. I well recommend this book to anyone interested in these fields. It also has a sufficient reference section to continue the discussion.

Ward and Brownlee present a good overview of recent thinking on "Habitable Zones". These are zones within planetary systems in which planets are expected to be habitable. The authors show that conventional thinking of these habitable zones - that liquid water be present on a planet surface, are really "Animal Habitable Zones". Since the concept was first created we have discovered life in places such as deep Ocean vents and kilometres below the Earths surface, life that would exist outside traditional habitable zones. They also present the new concept of Galactic Habitable Zones, parts of the Galaxy where we would expect to find habitable planets. These places require "metal-rich" solar environments: with sufficient heavy atoms to create terrestrial planets (and Gas planet cores), a lack of lethal Hypernovae and Gamma Ray Bursts.

However recent work may imply that GHZs are more common than the authors had presumed: work on Supernova 2006gy may imply that metals were more common in the early universe than previously thought. This would allow life more time to evolve. In fact, our Sun might be a relative newcomer, and the risk of GRBs may be smaller than originally thought.

While Rare Earth is good in its coverage of the issues, it pushes its thesis that conditions for life must be 'just so' a little too hard. It gives examples of biological feedback mechanisms that sustain complex life on Earth: greenhouse gases that keep the climate stable despite rising insolation, etc. However it considers Earth unique when it views features that threaten life on other planets : asteroid impact rate and the presence of Jupiter, lack of a moon to stabilise spin (and climate). That Gaian mechanisms might arise on other planets to solve these problems is not considered, even though its accepted that lower forms of life could survive these and bring about such stability. Everywhere it suffers the problem of 'changing one variable' and seeing only bad effects, of not spotting potential alternative 'survival' mechanisms that may evolve.

The chapter on Plate tectonics is very interesting. The discovery of the importance of geologic feedback cycles was one of the highlights of Earth systems science in the 1990s, and is very important to life on Earth. They show that, without the effects of tectonics, we would be unlikely to have our oxygen atmosphere, and hence complex life which depends oxygen for a high metabolic rate. This would be a major problem in Ocean planets as proposed by Leger:2003.

However, while Earth is the only planet in the Solar system with tectonics that we know of, the arguments made that tectonics would be universally unique to Earth are weak. Why should other planets of Earths mass or higher not involve tectonics? Also, I think that rather than tectonics being a requirement of life, it is an example of a deeper mechanism: thermodynamics, driven in this case by radioactive heating of the Earths mantle. Sufficient energy and dynamics are required to keep a planet alive, to cycle nutrients back to the biosphere.

On other planets different dynamical mechanisms may come into play. For a larger planet, asteroid impacts would not be fatal; On ocean planets, in the range 5-10 Earth Masses, with perhaps 100km of ocean we may see asteroid impacts as a cycling mechanism: heating the oceans or 'core' (a deeper layer of Supercritical water, for instance); convection bringing nutrients and heat from the planets Interior. On such planets (perhaps several AU from a Sun), such heating may provide more energy that photosynthesis for life.

Also worth considering is the factors behind the timescales: why does complex life take 3 billion years to develop on Earth? What sets the rate of evolution and adaptation, of life and of climate?If any given planet in a hypothetical solar system is sterilized every 100 million years, say, what is stopping complex life developing a civilization and migrating to other planets in that timescale ? the possibility of microbial life on Earth being seeded from Mars is discussed. If civilizations can evolve on other planetary systems in 100 Million years rather than 4 Billion, then a lot of their arguments become moot.

Prof. James Kasting also has an excellent review of Rare Earth here. Involved in a number of the discoveries described by Ward and Brownlee, He covers a number of these facets in greater detail than I have. On magnetic fields, he thinks it likely that terrestrial planets would have Earth-like magnetic fields and that this is not an issue; he stretches the point however when he says that the periods of magnetic field reversal, in which Earth had little magnetic field to protect life, shows that magnetic fields are not necessary. While Earth could do without these fields for short-periods, and life obviously survives the higher degree of cosmic radiation, sputtering processes would lose Earth its atmosphere if we had no such fields long-term.

Overall, I recommend this book to anyone interested in why there is, or is not, abundant complex life in the Universe. While it does not have the definitive answers, it summarises the questions and issues in a comprehensive but very readable fashion, and is a wonderful starting point to the state of the art.