Rare Earth (2000,Copernicus Press, New York) begins as a challenging book from its introduction which defines the Rare Earth Hypothesis as commonness of simple life and the rarity of complex life in the universe. Peter D. Ward and Donald Brownlee claim that their hypothesis will reverse the decentering trend, namely that our earth is not the one planet with life but one of many. A renewed perspective of the long-held belief in the uniqueness of earth and its life.
Among the number of argued reasons for the unity of earth, with respect to complex life, are: earth’s proper position in space, a core at the earth’s center, ambient surface temperature, moon’s perfect distance from the earth, and so on. There is much information both in figures and in the form of historical evidences and reconstruction. All the facts indicate the scholarship of the authors who are bent on proving the rarity of our planet as the sole carrier of complex life.
What makes the book, and the hypothesis, debatable is the weight of the proposed rarity factors in relationship to the mainstream cosmology. Offering details of earth’s phenomena only confines one’s vision to our planet, as one is naturally inclined to do. It does not refute the numerous possibilities of such phenomena on other planets, lots of which might be enjoying the same or even better living conditions somewhere in the remote space. Should we consider earth rare only because we are unable to know about other instances? All the observable part of the universe (Moon, Mars, Europa etc.) that the authors mention for comparison purpose is indeed a negligibly small part of Carl Sagan’s or Stephen Hawking’s universe. The Drake Equation, used to predict the possible number of civilizations in our galaxy, is confined to a single galaxy, our Milky Way. How many such galaxies are there in the universe? The authors seem to be assuming too much in adding their own factors to produce a modified Drake Equation. Nevertheless, the founders of Rare Earth Hypothesis grant that many unknowns exist (structure of solar system, biogeochemical processes affecting the origin of life etc.) in the way of accepting Rare Earth Hypothesis at this stage of our knowledge.
Rare Earth is informative and challenging. It has a special appeal for those who tend to see intelligent life, as of humans, as something unique. Equally, it captures the interest of those who have accepted the main stream cosmological view on the existence of life in the universe. Most importantly, Rare Earth is a book that motivates to get tuned to all possible sources of information on the issue. Just a few months back, in March 2005, NASA’s scientists directly detected light reflected by two extra-solar planets. According to the news, at least 130 stars outside our solar system have been shown to have orbiting planets. A curious mind can hardly keep from thinking what number of planets might be in the whole universe. In the context of such, and possibly coming, discoveries we might ask ourselves Is our life indeed a rare phenomenon?