To quote:

A scientist seeking to investigate the spectral signature of a life-bearing planet does not have many options to choose from. Just like Sagan in 1993, Montañés-Rodriguez and Pallé focused their investigations on the only such planet that is known to date – our very own Earth. Since no planet is as well documented and researched as Earth, this would seem at first glance to make things relatively easy. After all, measuring Earth’s spectrum from space is precisely what Galileo did in 1990, and there are satellites in orbit around our planet that are capable of making similar measurement. Surprisingly, however, data collected by spacecraft and satellites simply will not do, explained Montañés-Rodriguez.

Full article here:

http://www.planetary.org/news/2006/1220_Scientists_Search_for_Distant_Life_in.html