I only just came across this, from 2011, but I have to respond. Note that I’m not an astrophysicist, or a biologist, or a chemist, or… you get the idea. Seth Shostak is an astronomer at SETI. But I’ll make my arguments and you can decide for yourself.
“Indeed, it seems a good bet to guess that at least a few percent of all stars are blessed with “habitable” worlds. That would tally to billions of life-friendly sites, just in our galaxy.”
The first part is reasonable enough. Recent research has indeed found many of our near neighbors have “habitable” planets. But check the definition a few sentences earlier of “habitable”:
“…worlds with solid surfaces at the right distance from their host star to sport temperatures amenable to the presence of watery oceans and protective atmospheres…”
So we’re jumping from solid, not enormous, and in the right orbital zone, to “life-friendly.” That’s a huge leap. As far as I know the debate continues on just how big the “habitable zone” is. Take a look at the table at http://en.wikipedia.org/wiki/Circumstellar_habitable_zone and note that recent estimates for both the inner and outer bounds of the region vary by 2x. That’s not certainty.
Estimates of the number of terrestrial (rocky) planets in the habitable zone in our galaxy range from 500 million to 150 billion, but again, it’s important not to read too much into “habitable” or “terrestrial.” All that means is that if you we’re there in a spacesuit, you wouldn’t collapse under your own weight, or sink into the planet’s surface, and a cup of water, at least somewhere on the planet, would neither freeze nor boil. That’s a long way from “plant some corn and build a cabin.”
Finally, all of this ignores the simple fact that given a rocky planet where the water is liquid at least some of the time, at least somewhere, assuming there’s water there in the first place, we don’t know how likely life is to spring into existence in the first place. if it does happen, we don’t know how likely it is to become multicellular, or any of the other hurdles on the way to becoming the Klingons or the Vulcans. Or us. One data point does not allow for reasonable estimation.
The article insists that there are hundreds of billions of rocky, habitable-zone planets in our galaxy. Well, my estimate of the likelihood of life coming into existence on a given planet is a trillion to one, and no argument I’ve heard can disagree except on the basis of incredulity. “A trillion to one? That’s absurd! How did we get here then?” The simple answer is that if we weren’t, we wouldn’t be here to debate this.