Now that the discovery of extrasolar planets, or planets around distant stars, has become relatively routine, scientists are now tackling the next step: finding life-bearing worlds. To do this, observers must know what signs to look for in the feeble light from these faraway planets.

CfA astronomers Wes Traub and Ken Jucks, in collaboration with researchers at the University of Arizona's Steward Observatory, have identified key signatures of life by studying Earthshine-the light of the Earth reflected off the dark side of the Moon. They found clear signs of water and an oxygen atmosphere, as well as tentative signs of plant life. Their findings give a clear indication of what "fingerprints" to search for when seeking life on Earth-like worlds orbiting distant stars.

Artist's depiction of our world when the Earthshine measurements were taken, enhanced to show the "red edge" from vegetation. Credit: John Walker's "Earth Viewer," Christine Lafon, Harvard-Smithsonian Center for Astrophysics).

To replicate the view that a distant astronomer would have if studying the Earth from another planet, Traub and his colleagues measured Earthshine from the dark side of the Moon. The team found that the spectrum showed strong evidence for water - a necessary ingredient for life as we know it- and for molecular oxygen, which must be continually replenished by the processes of life to remain in the atmosphere. They also found features that suggested the presence of chlorophyll, indicating the existence of land plants.

The team noted that our home planet has maintained an oxygen atmosphere for the past two billion years, and has shown the chlorophyll signature since the first land plants evolved 500 million years ago.