The Harvard scientist even dismissed the future Kepler announcement, pointing out that upcoming reports of habitable exoplanets would be few and far between. "Extrasolar systems are far more diverse than we expected, and that means very few are likely to support life," he said.
Both Right and Wrong
So what can we learn about the disparity between Vogt and Smith's opinions about the potential for life on exoplanets, regardless of how "Earth-like" they may seem?
Critically, both points of view concern Earth-Brand™ Life (i.e. us and the life we know and understand). As we have no experience of any other kind of life (although the recent eruption of interest over arsenic-based life is hotly debated), it is only Earth-like life we can realistically discuss.
We could do a Stephen Hawking and say that all kinds of life is possible anywhere in the cosmos, but this is pure speculation. Science only has life on Earth to work with, so (practically speaking) it's pointless to say a strange kind of alien lifeform could live on an exoplanet where the surface is molten rock and constantly bathed in extreme stellar radiation.
If we take Hawking's word for it, Vogt was completely justified for being so certain about life existing on Gliese 581 g. What's more, there's no way we could prove he's wrong!
But if you set the very tight limits on where we could find Earth-like life, we are suddenly left with very few exoplanet candidates that fit the bill. Also, just because an Earth-sized planet might be found in the habitable zone of its star, doesn't mean it's actually habitable. There are many more factors to consider. So, in this case, Smith's pessimism is well placed.
Regardless, exoplanet science is in its infancy and the uncertainty with the "is there life?" question is a symptom of being on the "raggedy edge of science," as Nicole would say. We simply do not know what it takes to make a world habitable for any kind of life (apart from Earth), but it is all too tempting to speculate as to whether a race of extraterrestrials, living on one of Kepler's worlds, is pondering these same questions.
The Right Ingredients
In summary, I'll be keeping a close eye on the developments in the field of exoplanet studies, it truly is a noble effort driven by the search for extraterrestrial life, but I won't be celebrating quite yet, for the reasons I outlined in a Facebook comment on the Discovery News fan page yesterday:
I'm still holding out for the news that reads: "Second Earth Found" - [this exoplanet] will have all the right ingredients: orbit its star inside the habitable zone, spectroscopic analysis will reveal a nitrogen-rich atmosphere, evidence of water, roughly the same mass as our planet and it will belong in a system with a couple of gas giants shepherding the outer system. Until that happens, the champagne remains corked.
Special thanks to Ed Butcher for bringing the Telegraph article to my attention!