NEW DELHI, April 18 -- Public perceptions of alien life, shaped largely by science fiction, often revolve around images of humanoid beings or caricatures of animals roaming a planet similar to our own. In the real world, scientists usually look for no more than microbial life; in fact, they look only for signs that indicate such life could exist away from Earth.

One aspect has been common to both worlds, though. As in science fiction, real-world scientists have long viewed rocky planets that are just the right distance from their respective stars (or "within the habitable zone") as being ideal candidates for supporting life. Given that life exists on Earth, is it not reasonable to suppose that any other inhabited planet will have conditi...