In many cases both the media AND scientists seem to imagine life as looking at least somewhat similar to life on Earth...
Methinks you're confusing the 'Search for Extraterrestrial Life' with the SETI project. Vastly different things.
While water is required for the original primordial soup, there is no telling if evolution may have evolved organisms out of a need for it.
Except it's hard to envision even basic metabolic functions without a solvent, and the physical and chemical properties of water make it by far the most likely solvent to allow life.
At least, life as we know it.
If we were just looking for life that was like us, you'd be right. We'd be unnecessarily limiting out search. And we used to do just that, looking only for life similar to our own. Fortunately, we've wised up and are no longer just looking for EarthLife Mk. II. :D
As an example, one of the more interesting places that we are investigating the possibility of life within our own solar system is Titan. Titan's environment is nothing like Earth's, with a much thicker atmosphere and a methane cycle rather than a water cycle. The details can be read in this article, but the basics are that the Cassini probe has reported that there is a large flux of hydrogen within Titan's atmosphere, much larger than can be explained by known chemical processes. In a hydrocarbon-rich atmosphere like Titan's, it is expected that UV radiation from the sun will excite the molecules in the outer edges of the atmosphere, freeing mono-atomic hydrogen. Some of this hydrogen will escape into space, and the remainder, without oxygen to react with, should remain at a relatively stable level. However, this is not happening. Quite a bit of it is migrating down to the surface, where something is removing it from the atmosphere.
The most likely explanation is there is a chemical process going on that we are as yet unaware of. However, it has been pointed out that the process is also consistent with an earlier prediction of methane-based life. This hypothesized life would combine hydrogen with a carbon compound to generate metabolic energy. And in the second chemical conundrum, models of Titan's atmosphere suggest that the carbon compound acetylene (a quite suitable compound for our theoretical lifeform to feed upon) should be produced in significant quantities by the reactions in Titan's atmosphere. And yet, our instruments have detected very little of it.
There are other, non-methane-life-based, explanations for the missing acetylene as well, so the likelihood of methane-based neighbors is still quite slim, but the possibility exists and we're not overlooking it just because they aren't water/oxygen based.
One of the chief methods being proposed for looking for life on other worlds (particularly ones in other solar systems) is spectral analysis of their atmospheres as they pass in front of their sun. Given a knowledge of chemistry, we can get a good idea if the combination of gases is one that is stable or if some other process (chemical or biological) may be affecting it. A planet with a lot of free oxygen, like ours, clearly has something going on to generate it.
“scientists may not recognize life when they find it.”
This is probably going to sound pretty stupid to a lot of you but...
Often I think about this. When we look at the human species... in comparison to the universe... we are the size of molecules that have existed for a few seconds... We think we understand to some degree what the universe is. I think we give ourselves too much credit on what we actually know and our comprehension of the universe.
I like to compare us to ants. The ant probably thinks it has its world figured out. But the ant has no idea that it’s on a planet which is the third rock from a sun in a solar system which is one of billions of stars in the Milky Way which is one of billions of galaxies. It has no ideal of time and space. How old the universe is. What makes up the universe. What the periodic table is. The ant has no ideal of all its fellow living life forms that have lived and do live on this planet. The ant has no idea of what we humans are. You can yell at an ant all day and try to explain all these things. It won’t hear you or comprehend your communications or realize that you even exist.
I often think... what if we were just ants (metaphorically speaking)? What if there was some species out there which compared to us made us similar to our ants? This species would be right in front of our face looking at us.... and we wouldn’t even realize it. Just like we are to the ants.
You can yell at an ant all day and try to explain all these things. It won’t hear you or comprehend your communications or realize that you even exist.
All the better. Because if one of them does realize you exist, it won't still understand you but might end up writing the Bible.
I bow to no man in my respect and admiration for science. I especially revere it because it does not depend upon faith but upon verifiable, empirical, demonstrable EVIDENCE. That being said, the belief among most scientists that there must be life elsewhere in the universe is just that - BELIEF - and nothing else. Upon what, excactly do they base this belief?
First, there is the anthropomorphic bias we have deep in our psyches. Hey, WE are so wonderful that there must be more of us somewhere in the firmament. It is not a stretch to say that some scientists believe this "religiously." But this is a false argument, because every single thing that exists in the universe is in some way unique. Why can't life be one of those things? The answer: it can; it absolutely can.
Then there is the famous, iconic "Drake Equation." Everyone involved with SETI (The Search for Extraterrestrial Intelligence) cites this formula, created by astronomer, Frank Drake, some 50+ years ago. His most ardent and steadfast supporter was Carl Sagan. Sagan deserved all the accolades he received for his popularization of science in his TV series, "Cosmos," as well as his wonderfully poetic and informative books, such as "The Pale Blue Dot." However, Dr. Sagan had tunnel vision when it came to SETI. He just couldn't believe that the thing we call "life" occurred only once in the history of time. As a result, SETI enthusiasts - professional and amateur - have been haplessly, desperately, and fruitlessly sending radio signals off into the ether for decades in hopes that something other than "I Love Lucy" will return in the cosmic mail. There are two distinct issues with SETI. One is the technological one of ever being able to communicate with something that speaks our electronic language; with our symbolism; at our frequency; and contemporarily with our brief moment in time - all VERY remote possibilities. The other is the almost religious nature of human beings to suppose that our defining attribute - life - MUST be a repeatable phenomenon, given the immensity of space.
But, getting back to the Drake Equation. Scientists love to point out all of these factors that seem to suggest that, statistically, there almost HAS TO BE something like us lurking out there somewhere. What these scientists fail to recognize (or acknowledge) is that the equation is biased. Drake (intentionally?) included all of the factors that emphasized the nearly limitless opportunities for life to arise - but left out the negating factors - one in particular. Drake conspicuously (in my estimation) omitted the most salient factor of all: the chance that life would have ever arisen here on the earth at all. Nearly all evolutionary biologists are in agereement that life likely sprang into existence on earth in one single event, and that from this all forms of life as we know them today, branched and spread. The fact that ALL life is based on DNA, and only DNA, strongly suggests that the phenomenon of earthly "life" began as a one-off combination of proteins and amino acids that, amazingly and fortuitously, replicated itself into daughter cells that possessed the same powerful attribute. And we were off to the races. There is not a shred of evidence that it happened more than this one time. If it did, it did not evolve and leave traces of itself today. ONE SINGLE EVENT out of quadrillions, quintillions, sextillions...of opportunities and it ONLY HAPPENED ONCE, as far as we can tell. The immensity of that number, in my mind, supersedes all of Frank Drake's other (convenient) factors, which clearly were selected to provide a pre-ordained result - a result that validated the faith of the true believers.
So today, the skies are constantly being saturated with signals and scanned for responses - responses that I am quite sure will never come. Sure, life MIGHT exist elsewhere. But there is nothing that implies it beyond the anthropic principle. Further, even if life DOES exist elsewhere, the chances of it evolving to a form that resembles us and our technology in any way is vanishingly remote. We are not likely to ever receive some prokaryotic morse code from many light years distant.
No doubt, this post will generate responses from indignant scientists who accuse me of being narrow-minded and having no evidence for my position. But, to quote Dr. Sagan, "Extraordinary claims demand extraordinary evidence." And I am NOT the one making the extraordinary claim. So far, I have not seen any more evidence for life in the cosmos than there is for a god sitting out there somewhere. Show me God and I'll believe in Him. Show me a space alien piloting a "flying saucer" and I'll believe it. Show me life from another planet and I'll believe that. Until then, such concepts are based on nothing but faith.
Forget about the Drake Equation for a moment and ponder this. The Solar system and our lovely home began to form from a cloud of interstellar debris about 4,6 billion 'years' ago. Life on this planet began maybe 3,8 - 4 billion years ago, the oldest common ancestor of us (maybe after becoming successful in destroying all competition) 3,5 billion years ago.
If you zoom out on these events far enough, then you would say that the Earth was crawling with life about as soon as it was chemically possible.
This still does not exclude the possibility of it being an extremely lucky one in a - insert enormously large number here - coincidence, but at least it suggests otherwise.
The origin and evolution of DNA (RNA really in the beginning) might in itself be explicable in a pretty straightforward manner, the 3-letter coding system might have been evolved from a 2-letter (base-pair) coding system.
But anyway, the 'failure' of the SETI crowd (I'm talking about the sky survey angle now) to detect a signal is pretty much guaranteed. The off-chance that we would detect one more than weighs up to the small probability of it being possible. Even if the Universe is teeming with life that having discovered ways to use EM radiation to communicate with, chances of us picking up a signal in the window of SETI looking for it is unimaginably small. Even our home galaxy is a very, very big place and signal strengths wane fast.