Over the past two years, two teams of astronomers have been using this effect to figure out what Earth might look like as a distant, extrasolar planet orbiting another star. By analyzing the light reflected off the moon during a lunar eclipse – light that has passed through Earth's atmosphere – they have detected gases in the atmosphere that indicate the presence of organic life on the planet.
If the teams' baby steps are any indication, the techniques they are developing may be able to detect evidence of organic life imprinted in an extrasolar planet's atmosphere – at least for rocky, Earth-mass planets orbiting stars relatively close to the sun – using large Earthbound telescopes.
"It's an exciting experiment – one of the few I've seen that I wish I'd thought of myself," says Sara Seager [well lah dee dah! -J], a physicist at the Massachusetts Institute of Technology who studies exoplanets and their atmospheres and who was not involved in either project.
"The Earth is our best laboratory; it's the only planet we know of with life," she says. "So we really want to understand what Earth would look like as an exoplanet far away."