The Cosmic Filter
Calculating how often alien civilizations might occur, has been the preoccupation of scientists for decades. The first person to really formulate such an answer was Frank Drake using his now famous Drake Equation.
The trickiest part of calculating this equation is determining the probability for each variable. Over the years, our knowledge of astrophysics and planetology has increased substantially. For the most part, the probability for the occurrence of life has increased along with the discovery of extreme forms of life here on Earth. Life has been found in the coldest reaches of the arctic and the deepest parts of the oceans. There is one type of archae-bacteria whose natural habitat is boiling water.
Discovery of planets within a few light years of our own star system has upped the ante even further. More surprising is the types of planets we are finding, including the recent discovery of 3 planets orbiting a pulsar. For a good peak at our stellar neighborhood, check out the well maintained Extra Solar Visions Site.
If life does manage to get started, it has a long up hill battle to survive. Assuming that a complex ecosystem manages to encompass a planet, it then has to contend with the possibility of periodic asteroid impacts, nearby supernovas, and deadly gamma-ray bursts.
Of all of the things most likely to extinguish life, one of the more menacing are the gamma-ray bursts originating from the collisions of two or more neutron stars. Radiation from such a collison is so powerful, that it has the potential to wipe out planetary life from a distance of 50,000 light-years. Lucky for us, such collisions only occur in our galaxy about every 100-200 million years. However, in a universe as large as ours, such bursts our detected by our instruments almost once every second!
We can count ourselves lucky that life on this planet has survived the past 4 billion years and is now we ready to leave the womb planet. I suspect we will also soon gain the ability to predict when such deadly gamma-ray burst will occur and get out of harms way when necessary. Simply positioning yourself on the opposite side of a planetary body as the incoming gamma-ray burst should do it.
Such collisions suggest the possibility that advanced space faring cultures are very rare, as most life gets irradiated before it evolves the ability to leave.