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You are totally correct. Life on this planet does not "prove" life on others. However the Drake equation gives us a prediction of how much life MAY be found elsewhere.

I did not spend 7 years of my life building and operating a radio telescope looking for SETI if I did not think that life (tool building life :)) was possible somewhere else in the universe.

But why do you feel that this life is likely, as opposed to merely possible? What is the drake equation and what possible assertions can it make about life on other worlds? Sure, I readily concede that life on other planets is *possible*, but is it probable? That's the question.

I mean, let's be honest. What is the probability of life from non-living matter (ie. abiogenesis)? Saying that it is 100% because we are here is illogical since biological origins are a total mystery. That is unless you're basing all of this reasoning on the philosphical assumption made by all of science, that being naturalism which itself cannot be empirically verified with respect to biological origins.

The problem is that the numbers most people enter for the Drake equation overlook some very fundamental facts about our galaxy. The very first entry into the equation, the rate at which suitable stars are formed, is MUCH lower than most people assume. To wit: Much as our solar system has a "band of life," or a region of space where life can exist, so does the galaxy itself.

Deep in the center of our galaxy, for example, is a large black hole. Any planets in this region would be heavily irradiated and subject to massive stresses that aren't exactly conducive to life. Move out of the core and into the "bulge", and your radiation problems die down, but gravitational interaction between the densely packed stars makes stable planetary orbits very difficult to achieve. Novas are also somewhat common in the core region which can't be healthy for nearby life bearing planets. Moreover, the vast majority of stars in this region are Population II type stars with very little metal, and are therefore not likely to support planets, much less life.

Move out of the bulge, and into the inner disk, and the problem reverses...you run into a plethora of metal-heavy but hydrogen-deficient Population I stars. Some of these are very old and have burned out their hydrogen, others have gone dwarf, and still others simply formed in this relatively hydrogen-free section of the galaxy and are distinctly "below average". A star with below average hydrogen content is far less likely to contain water-bearing planets, which is practically a requirement for complex multicellular life forms. Tack on to that the fact that hydrogen-deficient stars burn much cooler than "normal" stars, and we can effectively rule them out as a likely home for E.T.

Moving into the central part of the disk, you find an equilibrium. This region is far enough from the maelstrom of the galactic core and bulge that the hydrogen hasn't been burned or blown away. Its stars are all moving in mostly stable orbits, and enough Population II stars have died in the area to provide plenty of metals. The primary phase of Population I star formation in this part of the galaxy ended several billion years ago, meaning that gamma radiation is relatively low and that this section of the galaxy is comparatively clear of debris. This is, of course, where we are and where any other life is to be found.

And if we move out further? The outer portions of our galactic disk are characterized by immense clouds of dust, many radiation spewing stellar nurseries, more unstable orbits than you'd care to count, and a population of stars that, in whole, is far too young to support complex, intelligent life. Even if a stable and life bearing planet could arise out there, the fact is that nearly all of them are too young to have done so yet.

I remember that when my astronomy professor was teaching this to us, he mentioned its effect on the Drake equation. By refactoring the rate of star formation to limit it to only the portion of the galaxy where complex organic life has a realistic probability of surviving, while leaving the rest of the numbers to the generally accepted probablilities, the equation stated that there were only seven intelligent species in our entire galaxy. Given the massive size of our galaxy, the probability of achieving any kind of meaningful two way communication with one of those seven are effectively nil.

Don't get me wrong. I run Seti@Home on four of my computers and hope against hope that we'll someday make contact with one of those species, but at the same time I'm a realist. The numbers don't lie, and they don't look good.