Though I’m not a scientist of any sort, I’ve had a lifelong interest in astronomy. The night sky, with the moon and the crowds of stars, has always filled me with a deep sense of wonder. I wanted to learn about these things, but the sources of knowledge—books, television programs, movies—tended to stress certain biases that repelled me: for example, the ignorance of thinkers from the past, humankind’s cosmic insignificance, or the absence of a deity. On the other hand, I did not wish to be taught by those who had a tendentiously pro-religious agenda. What I sought from scientists generally, astronomers in particular, was rigorous objectivity, lucid thought and theological neutrality. I finally encountered these qualities in Rare Earth, written by Peter D. Ward and Donald Brownlee. Their non-fiction book challenges the common assumption that the universe teems with complex life-forms (intelligent or otherwise). They argue that simple microbial life is probably common; but that complex life is rare—so rare that it might exist nowhere but on this planet.
The attainment and the continuation of complex life on Earth required a large number of critical factors that might be unrepeatable. For example, our sun (a less typical star than is often supposed) needed to have the right mass and a long enough lifetime, and our planet could not be too close or too far from it; we needed a large planet, like Jupiter, to minimize the impacts of comets and asteroids; our planet needed an ocean, but not too much or too little, plus a moon to stabilize its tilt; even our position in the galaxy is crucial, as is the type of galaxy we are in. These and many other factors were necessary for the successful start and prolongation of complex life on Earth. The theme is summed up by the authors:
‘Perhaps in spite of all the unnumbered stars, we are the only animals, or at least we number among a select few. What has been called the Principle of Mediocrity—the idea that Earth is but one of a myriad of like worlds harboring advanced life—deserves a counterpoint.’
The counterpoint which the authors provide is conveyed with an admirable combination of modesty and force. Moreover, Ward’s and Brownlee’s book has helped me see more clearly the greatness, the beauty, and the mystery of creation.
I read Rare Earth when it came out in 2000. Twelve years later, I contacted one of the authors, Professor Peter D. Ward, and asked if he still stood by the arguments presented in his book. He assured me that his and his co-author’s minds had not been changed by any recent discoveries. Whether or not that will hold up in the future is an unknown. But for now, Ward and Brownlee seem much more convincing to me than the more celebrated Carl Sagan ever did.