Today I saw this article and a funny coincidence struck me: on the day this article was posted, rd95 and I were on a long drive, and one of the podcasts we listened to was the 9/27 episode of Point of Inquiry, Lee Billings on the Search for Life in a Silent Universe. http://pca.st/0wxk
The premise of this article is that we may not be finding other intelligent life because it is buried in planets and moons like Enceladus with subsurface oceans. If intelligent beings evolved on such a world, would they even know the rest of the universe exists? When taking these frozen over oceans into account, we effectively move the boundary of the habitable zone of our solar system out potentially as far as Pluto. I brought these points up while we were listening, and now I'll pose some questions to you:
What are your thoughts on Fermi's paradox?
How likely do you think life might be on subsurface ocean worlds?
Where else might we not be thinking to look?
If by "intelligent life" you mean lifeforms either A) not of sufficient intelligence to broadcast detectable radio/light signals or establish surface-level architecture (at least, within our solar system) or B) of an intelligence so advanced that they use a method more advanced / less traceable than light signals. Maybe they'd use neutrino beams, or maybe they've hacked quantum entanglement. Or perhaps they're just hermits.
Otherwise, I guess the most "intelligence" you could hope to find is something akin to dolphins. Anything of a higher level of intelligence would have deduced that their world is spherical in shape, and we can venture a guess that some of them would've had enough curiosity to start exploring upwards. The most I'm expecting to find in our own backyard is complex organic molecules. Even microorganisms would be a shock.
But for other solar/planetary systems, well, that's where things start to get interesting. Especially with how common extra-solar systems are, apparently (thanks, Kepler mission!).
Oddly enough, I ran into Alan Stern yesterday. He was giving a quick speech at a memorial service for one of the space engineering community's greatest minds that was recently laid to rest. :(
Edit: Ah, right, the other questions:
1. Re: Fermi's Paradox: What incentive would an alien race have to make their presence known to us? Benevolence? Were I an advanced alien, I would let Earth keep incubating without interfering until they'd proven themselves deserving of assimilating into any existing galactic order. Granted, this is all presuming intelligent aliens exist, and I'm not sure that they do.
2. Very likely, but not in our own backyard. Now that I think about it, I'll bet that the challenge of space travel is even more daunting for liquid-dwelling lifeforms, because liquid (in general) isn't as compressible as gas, and compressibility is handy for transporting finite resources during space travel.
3. When we develop the imaging capabilities, we should look for alien space stations in orbit near the cloud tops of large gas giants with large magnetic fields. A strong magnetic field offers protection against solar flares, cosmic rays, and nearby supernova explosions.