Once it was asked in whispers, or with winks. The timid among us, though undeniably curious, feared raised eyebrows. Jokes about little green men. Who could take such a question seriously, yank it from the misty realms of science fiction and drop it under the searchlight of science? Well, our national space agency, for one. What's more, NASA seems pretty confident these days about the answer: Astrobiology, as defined on an official agency Web site, is "the study of the living universe."