No wonder these systems hold such fascination for scientists, for everyone! In October 1999, Science focused an entire edition on planetary systems. It featured a dozen papers on the topic, including one entitled “Expanding the Habitable Zone.”1 The habitable zone is that special region within a planetary system where conditions might be right for life to exist.
The point of this article and several others is that planetary systems may be more hospitable for life, i.e., the survival zone might be wider, than design theorists suggest. The point behind the point seems clear: Maybe the divine Designer isn’t really as obvious or necessary as recent design theory suggests.
As I look at the data on which these articles are based, I see the arrow of implication pointing in a different direction. Worldview assumptions do, indeed, influence our interpretation of facts, and a statement of these assumptions can be helpful. One “given” behind the notion of a wider habitable zone is that life requirements are extremely simple and few—liquid water, radiation protection, and an energy source—and that wherever these requirements are found, life can or will be found.
This assumption has been fueled by discoveries of bacteria in virtually “impossible” environments, such as two-mile deep gold mines and deep underwater steam vents. At the same time, planetary formation computer models demonstrate that deep underground pockets of liquid water (5 to 25 miles below the surface) may be more common than researchers previously speculated. In light of such findings, the chances for “chance” life seem significantly improved.
Scientific rigor, however, demands a closer look, a more careful and thorough evaluation of the data. A less “wishful” consideration of the data may yield a different conclusion. For starters, life in extreme environments on Earth depends on prolific and diverse life everywhere else on the planet. Taking away abundant life in ideal environments effectively destroys the possibility of life in the extreme environments. For example, the life found deep in gold mines and underwater steam vents relies on carbonates and oxygen compounds produced by photosynthesis at the planet’s surface. Only as efficient, long-lasting plate tectonic activity drives life and its nutrients underground can this deep-level life exist.
We know, too, that the more extreme the environment, the smaller the organism's body size must be for survival and the slower the metabolic reactions. These characteristics, in turn, diminish the capacity of such life forms to repair any molecular damage and to adapt to environmental change. In other words, life in extreme environments is microscopic, very primitive, and prone to rapid extinction.
A second point is that the more extreme the environment, the more difficult becomes the self-assembly problem. Even in the most ideal environment, the odds against life’s spontaneous generation add up to a virtual zero.2 The odds certainly cannot be expected to improve under severely hostile conditions.
The popular assumption that life needs only water, radiation protection, and energy flies in the face of accumulating data. The science journals of the past decade have identified more than a hundred characteristics of our solar system that must be fine-tuned for life to survive on Earth—once it is here.3 The more advanced the organism, of course, the higher the degree of fine-tuning its survival requires.
Given the range of sensitivities, I see the basis for proposing a number of distinct habitable zones in our solar system (or any other). The broadest zone would accommodate primitive, microscopic, low-metabolism, short-lived organisms. The narrowest zone would accommodate advanced, large-body, high-metabolism, long-lived creatures.
The test of my position lies in what happens to the list of characteristics requiring fine-tuning. If my view is correct, that list should continue to lengthen. If the naturalistic view is correct, the list will shorten. Thus far, at least, the trend line supports divine intervention.