Ah, yes, the highly publicized, if not hyped, search for planets with life 'out there'! A promotor's dream, because how can you ever falsify the idea that we're not in this alone? NASA and other funding-hungry semi-sci-fi agencies continue to drop hints about habitable planets and so on. You know the patter, and the promises almost as flagrant as those of 'precision' genomic medicine.
Well, forget the hype, promise and 'artist' impressions, and let's take a little look at what is in a sense being promised.....
First, let's go back to Aristotle's time and before, and imagine space as a crystal sphere in which the Earth is centered. That is, what we can see is the encircling sky dome. There we peer into the stars, galaxies, and so on.
Now let us suppose that there really is 'life' out there, and as a short-cut for our purposes here let's further suppose that what we mean by 'life' is living organisms that are advanced enough to use and/or transmit electromagnetic waves strong enough to reach Earth detectably. Now, to make the SETI (search for extra-terrestrial intelligence) effort easier, let's imagine that these particular emanations can be discriminated from all other sources of electromagnetic radiation in the skies, like light from stars, microwave background from the Big Bang, and so forth--that is, so we can specifically visualize the life-signals' beams by their being green. I pick green so we can think of them as being (intentional or not, but at least direct evidence) from the Little Green People (LGP) that are out there--if any of them are. That we exist suggests that thinking that life of interesting sorts exists elsewhere is not particularly surprising, and it must have many forms we might not even recognize.
The figure very schematically suggests what I mean. The ring around the Earth represents what we can see today. Again, we assume we can discriminate the 'green' radiation from all the other radiation coming in. The length of each arrow represents the length of time the civilization out there was sending a signal, and let's allow these to be for millions of years (note I'm being very generous: for example, any human-sent signal emanating from Earth would only be about 100 years' long, since radio signaling is only that old). The place of origin will usually millions or billions of light years away, way, way, way beyond this figure, and the signal has taken that number of years getting here, as it's doing just now when we see it. After the end of an arrow, that civilization was no longer sending, probably no longer exists, but we don't know that until the end of the signal arrives. Before the arrowhead, the civilization hadn't developed appropriate signaling technology, so we can't know it is (or was) there. Incoming signals outside of that ring are not yet visible to us--we don't know they're on their way here and have no way to know those LGP civilizations even exist.
The green-dappled sky
OK, envision our celestial globe, with its hundreds of billions of stars in the Milky Way, and the hundreds of billions of similar galaxies we know are in the heavens, again, just considering our 14 billion year old universe's visible horizon, which is I think tens of billions of light-years in diameter. If life is here on Earth, why not all over the place out there? There are similar environments, presumably, in countless planets. At any given moment, we would see green dots on the celestial sphere wherever an LGP signal was incoming.
So, let's say, just to be as favorable to the LGP search as possible, that we see a million such dots. That's not implausible, and let us assume, again to be generous, that each emitting population actively disseminates electromagnetic radiation for a million years, from the time they developed the technology until they evolved on to extinction or disinterest or whatever.
This should mean rich hunting grounds. But I think it does no such thing.
By far most, if not essentially all such LGP emanations will be from more than a million light years away from Earth. That means that by the time we can detect them, the emanations will have stopped and the people probably become extinct (many reasons, such as their local star blew up, they evolved to some other species, they damaged their environment beyond sustainability, they overpopulated themselves into starvation....). And of course, we have no idea just by seeing a dot whether it's one of those, or one from a still-living source.
Even if such a source were still viable, and even if we guess right about which one that may be at the time we mount an expedition or sent a 'Hello!' message, it would likely take 'us' so long to reach them, with either generations of astronauts in transit or just a radio signal, that the LGP would be done-for by the time we or our signal got there. A manned expedition would take hugely more generations than have ever existed for our species, of course. And, of course, no matter how clever they were, the LGP wouldn't be able to detect us for millions of years after we sent our message to them, assuming they were still alive at that time so distant from when we received their emanation. And, of course, we'd be extinct or would have polluted the Earth out of life, by the time any such contact occurred.
So, it is essentially hopeless to wave around vague suggestions of finding 'life in space'. In fact, most of the universe isn't just a few millions of light years away from us, it's billions of them. Even hinting at connecting with LGP borders on fraud by those who want our attention, and funds, to explore these possibilities.
The hopeful (sort-of) side
However, there is, surprisingly (sort of), a hopeful side. That's because if there in fact are LGP out there, most of their signals won't be visible to us at any given time. Millions or billions or even trillions of little green dots may be approaching us from any or all directions, but just haven't gotten here yet. That would only be expected if we can see a million of them at our particular time today.
Out of all of those incoming signals, maybe some are close enough to us to constitute something to be excited about....except that once we see their blip, it will at best have been sent at the beginning of their signal-generating lifespan and we'll have no idea how long ago that happened, and hence whether they're still there and sending, so we can't know how long the blip will keep appearing, and most of the time the signal will be millions if not billions of years old, and as above, the senders long-gone.
If some movie or video game outfit develops a real tele-transporter or wormhole traversing system, we might dream of going 'there', wherever there is. Even then, we'll have to choose which of the million green dot sources to travel to, and just hope the senders are still there. Of course, they're not really green, so choosing light blips to pursue won't be easy!
There are enough fascinating things about space, and enough real, not imaginary, needs right here on our green earth. We should keep fantasies to the world of fiction.