How closely do you think a planetary environment would have to resemble that of our Earth in order for life to exist there?

This question was inspired by the discovery of a rocky planet orbiting in the habitable zone of a planet just 42 light years from Earth. If you have time, be sure to view the slide show showing exoplanets as envisioned by space artists. It’s awesome viewing and the captions provide additional depth on our current understanding of such distant extrasolar planetary systems. The closest potentially habitable planets are the twin superterran exoplanets, Gliese 581 g and Gliese 581 d, both a mere 20 light years away.

If we could travel at the speed of light (670,616,629 MPH), we could get there in just 20 years. Unfortunately, the fastest man-made crafts to date are Helios 1 and 2 solar probes, which loll along at a leisurely 157,078 MPH. At this speed, a trip of 20 light years would take 8,539 years give or take. If we’re looking for a new home, we do need to find one in what, for humanity, is the habitable zone. But is all life so constrained?

Given the diverse ecosystems life has been found in on Earth, why should we assume all life must require the zone humans we find habitable? Life exists beneath the ice cap on Antarctica and around volcanic vents on the ocean floor in strongly acidic water that is well above the boiling point at atmospheric pressure. If life has managed to adapt to such extremes on Earth, why isn’t it possible life can exist on gas giants outside what we commonly assume to be the habitable zone?