Actually, the possible environments are being expanded in types and qualities. Someone suggested there are microbiotics in the atmosphere of Venus, and who was left to say "No way!"? Europa, Titan, Mars, all have conditions not so different from where extremophiles have already been discovered on earth. Instead of the statgement as it was written should have appeared something like it becomes clearer and clearer every day that more and more places in the universe could meet the conditions required for at least the most rudimentary forms of life.
The exploration of Mars and Europa will be very interesting. Perhaps they have life, perhaps not. I'm betting that life is rare in the universe, but only on intuition, not on evidence.