NASA's Kepler space telescope is finding lots and lots of extrasolar planets. But how many might support intelligent life? And, is there a "sweet spot" in the galaxy where SETI astronomers should aim their telescopes?
I'd say we have already stumbled across that sweet spot three decades ago, but more on that later.
First, we need to understand what are the prerequisites for the evolution of life beyond microbes. Because we only have one example - Earth - extrapolations are dicey. For example, if completely water-covered worlds are more common than estimated, chances for advanced surface life go down. What's more, only surface dwellers could develop technological civilizations.
Fundamentally, we do know that heavier elements, such as carbon, oxygen, and nitrogen, are needed for building planets and the complex molecules for life. And, we need a star - as well as a planet - that is stable over billions of years to allow for evolution.
Factoring this in, along with a host of other variables, Michael Gowanlock of the University of Hawaii and co-investigators ran an extensive computer model of our galaxy to look for the legendary habitable zone where intelligent life has time enough to evolve without being destroyed by cosmic catastrophes. They made a three dimensional model that estimated habitability as a function of distance from the galaxy's center, as well as stars above and below the galaxy's pancake-shaped disk.