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3rdvogon · #65 (**permalink**) 18-May-2007, 06:38 PM

A DIM I think the problem always comes down to the basic argument of belief/guess against fact.

Now I suspect that possibly 400 stars in every 500 has at least one planet capable of sustaining some form of life.

I also believe that on not all planets capable of sustaining life that life has actually developed. I reason that having the conditions to sustain life in some marginal habitats is more common than having the conditions in which life can get started. If there are only a few locations on a planet where the conditions are suitable for life to get started then the number of opportunities for the necessary chemical events to take place to get life started will be few also. In that case it might never happen during the lifetime of that system's star or it will take a lot longer than it did here on earth. so maybe on only 300 planets in 500 star systems does life actually get started.

Where life does get started in the majority of cases it may remain microbial. Microbial life is very successful and because of its high mutation rate is very good at adapting to changing conditions. Being multicellular is not always the greatest way for life to spread and multiply. my Guess is that on only 100 planets on every star system there is multicellular life.

The emergence of a technology using species is not an inevitable consequence of multicellular life. Intelligence has a biological price to pay. To put it simply memory and processing power place a resource burden on an organism for which if there is no immediate evolutionary payback will normally result in extinction. Evolution cannot have foresight it cannot be a long term investor and anticipate that knowing how to build power plants might be useful for an organism. if a mutation does not produce an immediate evolutionary dividend then it will die out. A lot of factors had to come together here on earth which produced a species with the right body-form potential, subject to just the right environmental changes at the right time whilst avoiding a major planet wide extinction event. On other planets which have been supporting multicellular life much longer than earth those particular factor may not have combined so that no smart tool using species has yet emerged. Therefore I would not be surprised if only one planet in every 500 star systems produces a tool using species and some of those may still be sitting round camp fires bashing rocks together.

Now all of the above is just my speculation of what might be the case - it seems reasonable enough to me though I am sure others will disagree but I will never stand up and say that HAS to be the situation.

Unlike 30 years ago we do at least now have proof that other stars do possess planets.

We are fairly confident that there are planets which contain water and which exist at close to the right temperature for life.

Beyond that we do not know about life, other than we have one example of where life occurred and that is here. In other words we have our sample or ONE and nothing more.

We do not yet have proof positive of the existence of any life (not simply some organic precusor chemicals) other than that which originates on this planet. As yet none of the meteorites that have been collected or any other samples have yielded evidence that is not heavily disputed. Therefore until there is agreement that a particular sample of something from somewhere, shows evidence of something that either is or once was clearly a living organism, that it can only have originated from somewhere other than this planet and therefore not be the result of some human contamination, then we must accept the cold reality that the Jury is Still Out when it comes to life anywhere else but here.

It is true to say that absence of proof is not proof of absence - but then again neither can we claim existence elsewhere on the basis that if we are here then there must be others somewhere. That is simply flawed reasoning.