Ward and Brownlee candidly admit, in the Introduction, to a prior belief that Earth is special. It might be thought that that would make no difference to their arguments, but it does because everywhere they assume that only Earth life on an Earth-type planet can support anything higher than microbes. The Goldilocks Principle is taken to the point of absurdity: on Earth, everything is "just right". Of course, Earth life has evolved to take advantage of Earth conditions. Nevertheless, it might be more convincing if they could find one or two little things less than perfect, but they dare not risk knocking the Earth off the pedestal they have constructed for it.
Naturally their beliefs lead them into logical contortions. Let us take their treatment of the planet Jupiter as an example. Their approach has been to treat almost every contingent fact of the Earth's existence, and life on it, as a necessity; so to them complex life, on an Earth-like planet, needs a nearby Jupiter.
* To them, Jupiter acts as a planetary policeman, who in Chapter 3 sends us the good stuff, i.e. small comets for water and rare metals, while in Chapter 10 keeping out the baddies such as planetesimals. That's very clever. How does it know which is which?
* Jupiter is praised for purging the middle solar system of planetesimals, but unfortunately it caused them in the first place. And the asteroid belt is still there as a source of damaging planetesimals and rocks.
* Jupiter prevented the creation of an Earthlike planet in Mars' orbit. So Jupiter could be viewed as a negative factor - making Earths rarer.
It seems that Jupiter-type planets are likely to be formed beyond the "snow line", where terrestrial planets will not form. Because of gravitational effects, they appear far more destructive than beneficial, but of course W&B don't want to say that.
Another frequent assumption made by the authors is that the limitations of Earth life must also be the limitations of life elsewhere. Here, life has evolved strategies to cope with, e.g., the temperature extremes found on Earth (by hibernation and estivation (i.e. sleeping over the summer), special "antifreeze" in the blood, etc). W&B assume, without logical warrant, that alien life would be unable to cope with greater extremes such as might be caused by (for example) a more eccentric orbit than Earth's, or a higher radiation environment.
On the positive side, there is a lot of science described very clearly, so on this basis I have awarded four stars. However, it is not for beginners - I suspect that readers without the equivalent of at least one A-level in Science will find it hard going.
There is a good index. Obvious mistakes are minimal, although there there is a particularly poor example on page 281 where Pierre-Simon de Laplace is referred to as "de Liplike".
Cohen and Stewart plausibly suggest a much wider range of possibilities for life than is simply assumed in this book. In any case, those who like to think will wish to read this book and assess for themselves the probability, not of planets exactly like Earth in solar systems exactly like ours, but where some kind of complex life might evolve.
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