It's a little game I play with myself, called: Spot the Alien Intervention. It doesn't have rules so much as premises:
1. The sun, solar system, terrestrial planets, life-itself are not unique; and represent the normal and usual evolution of (Type G, Early F, Late K) stars in the galaxy and in the universe.
2. Life evolves spontaneously wherever there exists sufficient molecular complexity in a sufficiently stable environment for biological replication to occur.
2a. Complexity of replication, over time, always results in the evolution of intelligence; the more complex, the more intelligent.
3. The older the star system, the sooner life, and intelligence, evolve in it.
4. With the evolution of life, and intelligence, comes inevitably, in time, the discovery of the means of inter-stellar space travel.
4a. The discovery of the means of inter-stellar space travel begins first with the evolution of an intelligence capable of understanding either (1) that inter-stellar space travel is possible (based upon such proofs as it might possess due to the carelessness of visible, or detectable, inter-stellar space travelers); or (2) that it might be possible under certain conditions, given adequate means of propulsion and protective encapsulation.
5. Inter-stellar space travel is, therefore, presumed to be a fact of life and a modus vivendi for all (Type G, Late F, Early K) star-systems older than 4.6 billion years old.
5a. The nearest star-system, therefore, in which inter-stellar space travel has evolved is Alpha Centauri.
Now I will quote a little bit from the article entitled Alpha Centauri, A Candidate for Terrestrial Planets And Intelligent Life (Last Update October '97 - new update currently in preparation), whose address I show below (but cannot make "live"--God damn the whizzes at Google Central who make editing one's blog insuperably difficult!):
"Alpha Centauri is a special place because it may offer life conditions similar to our solar system. A star must pass five tests before we can call it a promising place for terrrestrial-type life (1. Main-sequence longevity. 2. Spectral type: G, Late F, Early K. 3. Stability. 4. Age: more than 4,500,000,000 years old 5. Heavy elements: at least 2% metals)....Comparing with our system we see that both Alpha Centauri A and B might hold four terrestrial planets each...and both might have one or two planets in the Life Zone where liquid water is possible.
"....It is interesting to note what NASA administrator Daniel S. Goldin declared in 1992: 'Imagine if spectroscopic analysis revealed a blue planet with oxygen atmosphere just 4 light years away orbiting Alpha Centauri. The demand to build a warp drive would start right away!' [See Premise 4a. above] When will we know whether there are really planets at [sic] Alpha Centauri? Actually, the Hubble Space Telescope is checking for their existence...."
Out of the mouths of nerds.
Now, as to the promised evidence of Alien Intervention: Note that the foregoing giddy little effusion was written eleven years ago (with its precious information about the probable number of terrestrial planets within what distance of both Alpha Centauri A and B), saying that our incredible Hubble Telescope was even then being trained on these almost certainly inhabited stars...And with what result? No one knows. In fact, it turns out that there was something--oh so slightly miscalculated about that Hubble Telescope focal length..from the very beginning....Oh well, they've sort of fixed it. Well, kind of. They still can't see any planets around Alpha Centauri A or B--But how about those Deep Field images!
http://homepage.sunrise.ch/homepage/schatzer/Alpha-Centauri.html