Here's how this book is supposed to start: "My fellow humans, we stand here today at the edge of a new age of cosmic discovery which will transform all of our lives. Recent breakthroughs have sparked a scientific revolution, and brought us to the point where we finally know how to search for life in outer space. Any day now, we may meet with success and find proof that we are not alone in the universe."

Today, our researches and ruminations are informed by much new information about the evolution and nature of both life on Earth and other planets, distant and near. Still, the question "are we alone?" Is one with no answer as yet. So, of necessity, this book is mostly about what we don't know. A book summing up everything we know about alien life would be quite short, containing only one word: "nothing". I've managed to add an additional 150,000 by following many elaborations of the question, both historical and modern, including speculative science, history, philosophy and fantasy.

The first popular science book devoted to the question of extraterrestrial life was written in 1686. In the preface to his Conversations on the Plurality of Worlds, the French poet and philosopher Bernard le Bovier de Fontenelle wrote:

"I've tried to treat Philosophy in a very unphilosophical manner; I've attempted to bring it to the point where it's neither too dry for men and women of the world nor too playful for scholars."

In his day scientists were still philosophers, science was still "natural philosophy", and belief in a cosmos full of planets inhabited by intelligent creatures was becoming widespread among European scholars. Like Fontenelle, I've been unphilosophical in places. Many scientific ideas and truths, to be expressed accurately, must be couched in endless caveats and qualifications. When I write popular science, there is always a little imaginary scientific colleague pouncing on my shoulder, telling me to clog the science at every turn with caveats, whispering in my ear "provide more detail" "show how we know that" and "don't you dare step out on that limb". I've tried largely to ignore that little monster, lest the book become too freighted with detail and fall from your hands.

One of the themes of this work is the long, often uneasy relationship between astronomy and biology, the two scientific fields which must get in bed together if we want to make real progress in understanding the potential of this universe to create life in other places. After a century of flirtation, they started going steady in 1960 with a tentative, insecure union called "exobiology". Then, after a 35 year courtship, they finally took the plunge in the late 1990s in a marriage called "astrobiology".

The way I see it, astrobiology is the newest name for an old quest. As such it is an interesting phenomenon in the evolution of our scientific culture. Here I will try to put the present moment - and the belief that we are hot on the trail of aliens, and witnessing the start of a new scientific revolution - in a wider frame, step back and have a look from a larger historical vantage point. To me, the study of ET life is as interesting for what it reveals about our own biases and hidden assumptions as it is for what it reveals about life in the universe. We strain the boundaries of good science when we extrapolate to the rest of the cosmos based on only one example of a planet with life. We come across many questions which are great fun to contemplate: Could there be a world ruled by intelligent plants? Life on a gas world like Jupiter? Planets which are much better suited for life than Earth? Sure. Why not? Such questions force us to refine our views about intelligence and evolution, and push us to define life in a universal sense, even though all we know is life on Earth. This topic pushes us right to the edge of our scientific knowledge and methodology: We can't proceed at all without making some provisional assumptions based more on pragmatism than evidence, and there are important aspects of the subject that can't be fully addressed right now by science.

Because I pay special attention to the limits of science, in a sense this is not strictly a science book but a work of natural philosophy. By using this term, I want to encourage a certain perspective on the science, an attitude where we keep ourselves honest by frequently questioning the framework of assumptions we use. I discuss some new ideas, currently on the shifting boundary between science and natural philosophy, that may be helping us to derive a less Earth-bound view of what it means for a planet to become alive.

Science is attempting a noble new assault on the question of our cosmic aloneness. But the question encompasses far more than just science. Astrobiology, I believe, is leading the way in helping the scientific community to, once again, think like natural philosophers, harkening back to a time when science was not distinct from philosophy, when the universe was not carved up into the turf of separate disciplines and sub-disciplines each speaking their own specialized language, and when even the lines between our study of the physical universe and our spiritual quests were not so finely drawn.

After I've lulled you into submission and taught you to respect my authority as a scientist, then hopefully you won't notice when, in the "Belief section", I start crawling farther out on various unsupported limbs, where the juiciest fruit is often found. Increasingly, I'll make statements that I cannot rigorously justify, but which seem, to me, true. In this section I allow myself more freedom to discuss my own beliefs and thoughts on the directions in which our science is, and should be, going. There is such a thing as scientifically informed intuition, and I rely more on this inexact tool in the last section of the book. My explanations and justifications are inevitably looser than those found in the Science section. I'm saying this now to give myself license, so look out.

Here I discuss our efforts to theorize about, and even communicate with, intelligent aliens living on planets circling distant stars. I also grapple with the widespread beliefs that aliens are already here studying us or perhaps even infiltrating our societies.

After I discuss some of the more fringe ideas about aliens which permeate modern culture, I speculate on some future possibilities. What might intelligence become, eventually, on Earth or elsewhere, and what are the implications, both scientific and spiritual, of these far future evolutionary possibilities for the ultimate role of life and mind in our universe?

David H. Grinspoon is a planetary scientist at the Southwest Research Institute in Boulder Colorado. His book Lonely Planets: The Natural Philosophy of Alien Life was published in November 2003.

Click for Part Two

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