15-30 Billion Earths: "Where is Everybody?"

That's the estimate of how many habitable earths there might be in our Milky Way galaxy alone, according to data from the Kepler space mission. There's an interesting visual overview of the research at http://exoplanets.newscientistapps.com/.

Which leaves us with the conundrum of why we've never heard from any sentient beings from anywhere else in the galaxy. If they exist and know we are here, are they just ignoring us? Or is it a case of a time mismatch between civilizations at different stages of technological progress? Or are we (and they) simply not tuned to a common radio frequency? Will we ever know? I hope so.

The same question led Enrico Fermi to ask the famous question "Where is everybody?", the so-called Fermi Paradox. More (much more, with many hypotheses) on that at http://en.wikipedia.org/wiki/Fermi_paradox. I particularly like the "too far apart" hypothesis.

Food for thought, anyway.