Terry,
I have never seen the logic behind the numbers in predicting liklihood of life.
Only possible things happen.
Think of it this way. Let's say there are certain baseline requirements for life to exist. For the sake of argument, let's define a couple here:
- Gotta have liquid water.
- Gotta have a stable heat source.
- Gotta have organic compounds at the surface.
Here's the important point: Unless you believe that a deity intentionally gave Earth these necessary features while denying them all other planets, then it means we got them simply because we happened to be in the right place at the right time. In other words, chance.
If something happens by chance at a given rate, and the number of opportunities is significantly higher than that rate, then it is likely for that thing to happen. For example, if a coin comes up heads one out of two times on average, then we can say confidently that if we flip it one hundred times, we are extremely likely to get heads many times.
If we accept that there is a non-zero chance of circumstances lining up to create an environment that supports life -- and we must accept this if we do not believe in the life-producing deity -- then there are implications in the same way as in the coin illustration. If, for example, there is a one-in-a-billion chance of any particular planet having the necessary features for life, but there are a billion billion planets out there whose component features are distributed uniformly, then it is extremely likely that there are many planets that are conducive to support life.
There are probably baseline requirements to life, but they may not be as difficult to come by as we imagine. We already know that there is water on Mars, and there is increasing evidence that the water was liquid in the past. There may in fact still be liquid water in some pockets. The feature "gotta have water" may be present in, say, 1 out of 50 planets. We have observed organic compounds on Titan (moon of Saturn), so we know that these can exist elsewhere. We know that it is possible for planets besides Earth to be close to stars (c.f. Venus and Mars).
If there is a one-in-a-billion chance that any particular planet has the right ingredients for life, and there are a billion billion planets (easily possible), then we can expect that there are about a billion planets that could support life.
The question of how many of those planets actually have life on them would work along similar lines. That is, again, unless you accept the life-deity magically and intentionally creating life on Earth, it means that life came about through natural processes which clearly have a non-zero chance of occuring in any environment that supports them. As long as the number of opportunities is significantly greater than the rate of occurance, it is statistically likely that it has happened many times.
SNG
