I don't know why this is even a topic of debate, considering many meteorites have been opened to reveal fossils of bacteria and other micro-organisms.
As for the rare earth hypothesis arguments (people who say the chance of other life-supporting planets is astronomically slim) that hypothesis is quite unsound.
First of all, all the underlying assumptions for the hypothesis are flawed (life doesn't have to occur under the exact same conditions as earth) and there have been discovered several habitable planets already, which puts the percent error for the estimations of the probability for finding a habitable planet well out of reasonable range.
In fact, there's a particular planet, whose existence needs to be definitively confirmed, that is only about 20 light years from earth. If
Gliese 581 g turns out to exist, then it puts the probability of finding any habitable planet in any particular solar system at around 10-20%. Considering the billions of stars, that's a large amount of earths.
In addition, there are other ways to "cheat" the habitable zone, as it were. If a planet of sufficient size is too close to the star, one of it's moons may be earth like, likewise if the planet is too far. The idea is that a planet just barely out of range might experience climate fluctuation itself, but could itself act as a buffer for its moons. In addition, consider a planet (like
Gliese 581 g) that is just too far of the habitable zone, yet several times the size of earth. This planet would doubtlessly have a dense enough atmosphere to trap sufficient amounts of heat to support life, without needing all the sunlight that an earth like planet would.
As you can see, there is an effective "extended habitable zone" that drastically increases estimates for potential life supporting planets. Kepler has compiled a list of all the 2,326 planetary candidates, over 1000 of which have been confirmed and 54 of which are predicted to lie in a habitable zone, and more than that lying in extended habitable zone.
For those of you who are interested: (If you go through these links and find the name Gliese reappearing, it is not because they are near each other. I don't know why many of the supposed habitable planets are in systems named after Gliese, but I found it interesting that there were more than three stars with the namesake Gliese that all had multiple proposed habitable planets.)
http://en.wikipedia.org/wiki/HD_28185_b
http://en.wikipedia.org/wiki/Upsilon_Andromedae_d
http://en.wikipedia.org/wiki/55_Cancri_f (This one exists in the orbit of a binary star.)
http://en.wikipedia.org/wiki/HD_85512_b
http://en.wikipedia.org/wiki/HD_85512 (Gliese 370, the star)
http://en.wikipedia.org/wiki/Gliese_581 (Gliese 581, the star)
http://en.wikipedia.org/wiki/GJ_667 (Gliese 667, a triple star. Like a binary star, in which one of the members of the larger binary, is itself a binary star.)
http://en.wikipedia.org/wiki/Gliese_876_b (This planet has a year of 61 days.)
http://en.wikipedia.org/wiki/Gliese_876 (The star of the above planet and her sisters.)