I must admit that I am not among the top zillion qualified lot to talk about this topic, yet I find pretty hard to control my emotions (not to post it). Quite often I end up seeing articles on search for ET Life, and the sort of "Norms" that they set as search criteria in the process - Oxygen, Carbon, Water, Earth-like planets, Sun-like stars etc. And I always feel that they have a great miss-interpretation on the "thing" called "life" terrestrial or extra.
I agree that my statement may sound pretty lunatic when it comes to life of planet Earth. We are Carbon based life forms, defined quite clearly, whereas everything else is regarded as non-life. This is greatly thanks to the common root (presumably) from which the life on Earth evolved. Unfortunately, the life elsewhere will take its own paths from its own roots (can be multiple roots too).
Hence we need to define life in more abstract and generic way in order to detect it elsewhere. At this point we have to recognize the most critical fundamentals of life. I think there are only few elementary features that made life sustainable on earth. Assuming that base minimum requirements can cause it, let's generalize them for any extra-terrestrial location.
1. Ability to multiply its own structure (be it chemical of any other).
2. Ability to apply mutations to the new generation at the correct rate [i.e. mutations strong enough to adapt to the rates of environmental transformations, yet subtle enough to retain the basic properties of the original]
In my opinion, any "thing" with above two features would sustain and develop into what can be called "life" by themselves. When I stated any "thing" I did not limit myself to amino acid, big molecules, any particular element such as carbon, or not even to matter itself. It can be any combination of "thing"s that exists in the universe. (Even in virtual universes such as cyber space – but let's leave that part for AI rights groups, one day, when they start appearing)
Self reproduction which I mentioned as fact (1) above, is a fundamental that happen in many processes. Rivers give birth to other rivers, rocks break into other rocks and combine also in enormous timescales. Stars explode and the subsequent nebulae give birth to more stars. Mostly, the reproduction happens with mutations, making the child different to parent. Nevertheless, an effective mutation is all about the extent or rate that it changes the original. Mutations capable of matching the exact speeds of the change of environment, will sustain, as the early amino acid or DNA did. If the mutations miss the beat of nature, the overall process does not sustain for longer periods such that they become something unique and progressing that we can call "Life".
On this basis we can state that all the other features which we consider as essential for life are just macro-scale composite features that are results of above two basics. And they are just defined (taken for granted) based on the life on Earth. Unfortunately, these by-products can be completely different elsewhere.
With right dignity to all the religious and meta-physical ideologies, let me leave aside the concept(s) that something lives inside the chemical bodies of living beings. The belief of soul/athmaya/vingnanaya(as in Buddhist philosophy) are beyond our ability to measure or debate. For that very reason it's not a good classifier for detecting life. If souls/vingnanaya drives humans and animals, what makes you think that trees, rivers and mountains don't have souls/vingnanaya? How do you prove?
On the other hand it is compelling to accept that every activity that living beings manage, do not ESSENTIALLY need any such driving phenomena, but can be explained purely based on underline chemistry of DNA and other building blocks of life. It's like AI where small deterministic programs get together in such a complex way to emerge [and pretend] as if they can think and decide. The huge ha-ho of soul/athmaya/vingnanaya can be something similar to a tribesman trying to explain an artificial intelligence system, imagining that some ghost is making the decisions. This point is a strong test against all the religions. I would leave them out of the debate here with the view that we cannot detect life based on something that is not detectable (hence escaping the charges on sigma Blasphemy for all religions).
There is another major question on life and non-life. That is the matter of reaction speed of the species. Imagine that there lives a life form which takes one million earth years to produce offspring (oh well!!!). The sluggish beings do their day-to-day actions in the durations of thousands of earth years. They are immovable and "dead as rocks" for the time span of a human life. Will humans detect it as life?? And then imagine an alien species who works so fast that so many generations pass through within a period of a mere milliseconds (don't expect them to have material bodies btw) They may build civilizations which lasts for less than a second. Can we detect them? We'd call them both "non-life".
Now this story does not have to be Extra Terrestrial. If we generalize the reaction time difference it can make us re-define many natural processes of our own planet as life. For some time, I've heard, that some people considered earth as a giant living being.
At this juncture, we come to a point of asking "what is this life thing anyway?", or "what right do I have to state that humans are life and rocks are not?". Both undergo chemical transitions. One can sustain its transitions in such a way that it converges and evolves towards some direction, whereas other's transitions are hardly progressive and say, at best, sporadic. Just that progressive evolution makes humans do things that classify them for "life". Wrong place at wrong speed for rocks unfortunately, as their changes nullify themselves and never develop into a complex living beings.
Slowly I'm closing in on a logical doom. "Life" which I generalized at my best to two basics, is itself a definition, a meta-symbol. That depends on the reaction speed of the one who defines it. A generalized picture of life cannot be defined. More general you are, it becomes clearer to you that the term holds nothing in it - or everything in it.
As per the perception few decades ago, many thought that aliens are like humans with ears noses and mouths etc (slightly deformed as their mamas had no folic acid pills during pregnancy :)). Today's scientists are trying to think out of this tiny box. How wonderful!!! ET life does not have to look like humans. It just needs water/oxygen and Mediterranean climate. Well, bravo chaps. You've just got out of the first box of a series which you don't know of.
But what else can they do? Our technology today is so weak that we're unable to detect even the water/oxygen based inhabitants of Mediterranean climate elsewhere. There can be so many various life forms in 3000 degree hot furnaces in other solar systems, or even in Sun. But how can we detect such a variation whereas we can't find our own ET cousins. So isn't it better to start at 30 degrees? I think it's better than just sitting and thinking about the magical variations of possible life forms all over the universe (which I'm so fond of doing).
However I do not know how realistic this search is. I do NOT believe that there is a "higher likelihood" for life to exist in earth like places. The story of Earth Life is unique enough for it to be "unlikely" to repeat elsewhere. It's not for the "likelihood" of life to EXIST that we choose these planets, but because we have higher likelihood to DETECT life there.
Attempts of search for ET life, is nothing different to early colonial archeology explorers in Indian sub-continent. The region had such long history that some archaeological sites were beyond detection with their technology. They detected what is obvious, while stepping over the treasures of later historians. Nevertheless their efforts made way for latter generations to detect such sites.
Nevertheless the journalists and scientists go wrong when they open their mouths. They need to state these things more generally and humbly. They ought to mention that "life" can exist anywhere in any form. They ought to (perhaps turn a little red and) state that there is not much meaning in this big term called life anyway. Then accept that they have no strength to dig all the depth of this universe but they are starting at their best point. And they're just little kids making sand mansions in the backyard of this universe. As a matter of some tragic fact, the extent of this universe can make humans much younger than that.
