At the Astrobiology Science Conference 2008 Nick Wolfe of the University of Arizona said yesterday that the best way to tell whether an exoplanet may harbor water — and life — is for us to launch a mission into space that will look back at Earth. Ever since Voyager I launched we've had a chance to gaze from afar at the homeworld, but for some reason we've passed up the chance. Wolfe said that's a critical oversight. As we search for new planets that might harbor life around far-off stars, it might be useful to know about what our own planet looks like from a distance.
The Earthrise photo (and check out the video if you really want to feel tiny) taken from Apollo 8 is one of the most famous space pics ever taken. Along with a few other nearly identical images, the shots are the only space-borne perspective that feature our pale blue dot from anything like a wide-angle view. This sort of thing is exactly what we need more of, Wolfe said. Imaging all of the phases of Earth (crescent, half, gibbous, full, etc.) from at least one lunar distance away would give us tons of info for what a world with continents, a dynamic atmosphere and water looks like.
The grand prize would be taking an image of the Sun's reflection on our oceans in polarized light. "That would give us a measurement of what the glint of sunlight on water looks like," Wolfe said, which could be used to determine whether planets are other stars have liquid water on their surfaces too.