NASA scientists have used the Deep Impact/EPOXI mission to look at Earth as if it was an alien exo-planet to see if they could detect life.
Read more: http://www.dailygalaxy.com/my_weblog/2009/06/nasa-blue-oceans-a-fingerprint-to-identify-earthlike-worlds-.html
They have also imagined what alien life on other planets might look like: http://www.dailygalaxy.com/my_weblog/2009/06/sky-whales--sci.html
