So astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA), in collaboration with researchers at the University of Arizona's Steward Observatory, decided to look at Earth, from space, in a roundabout way. Sunlight is reflected off Earth and hits the dark portion of the Moon when its not full. This light can be seen from Earth as it reflects back. Its called earthshine, and some skywatchers look for it at certain times of the month. Sensitive telescopes can pick it up more easily.

The study found clear signs of water and an oxygen atmosphere, as well as tentative signs of plant life, the researchers say. Their findings give an indication of what "fingerprints" to search for when seeking life on Earth-like worlds orbiting distant stars. That is, assuming, there is any life that follows the same rules as life on Earth.

"Our research is paving the way for future missions like the Terrestrial Planet Finder," a planned NASA space observatory, said Smithsonian astronomer Wes Traub. "Hopefully, within the next 10 years astronomers will be able to confidently say that some as-yet-undiscovered planet is a living world like our own."

So far, astronomers can only detect Jupiter-like planets around other stars because such planets are large and create strong gravitational signals. However, as technology continues to improve, astronomers will be able to locate Earth-like extrasolar planets later this decade or early in the next, if they exist.

To replicate the view that a distant astronomer would have if studying the Earth from another planet, Traub and his colleagues used the nearby Moon as a mirror. Using the Steward Observatory 90-inch telescope at Kitt Peak, Arizona, they measured both the light of earthshine from the Moon and the light of the Moon itself, then corrected the Earthshine to determine how the Earth would appear to a faraway observer.

They compared this measured spectrum to a model created by Traub and CfA's Ken Jucks.

The team found that light reflected from Earth shows 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 latter showed up as bright reflections in the far-red region of the visible spectrum. This "red edge" is a signature of chlorophyll, scientists say; it appears green to us only because our eyes aren't very sensitive at the red end of the visible spectrum.

The team also suggests that chances for finding life-bearing worlds are improved because the signatures can develop early in a planet's history and last for a long time. Our home planet has maintained an oxygen atmosphere for the past 2 billion years, and has shown a "red edge" since the first land plants evolved 500 million years ago, scientists say.

"If someone out there is watching our solar system," Traub points out, "they could have detected plant life here long before any intelligent life appeared."

These measurements complement those made by the Galileo spacecraft during a 1990 fly-by of Earth. Instruments aboard the spacecraft also found evidence of gaseous oxygen and land plants.

However, the Galileo measurements were made while it was close to the Earth and show conditions only in limited areas of the planet's surface. Studying earthshine, on the other hand, yields a spectrum integrated over the entire visible surface of the planet, which matches the view that would be available to a distant astronomer in another star system.

The measurements by Traub and his colleagues were reported in the July 20, 2002 issue of the Astrophysical Journal and were announced in a statement Wednesday.