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The hunt for other Earths

Astronomers have speculated for decades about the possibilities of searching for life in other star systems. Now they're preparing to do it. To help them know what to look for, two astrobiologists have made a computer-based simulation of how organic life changed Earth's atmosphere during crucial stages of life's evolution. Although there's no guarantee that alien worlds undergo similar changes, the study typifies the action-oriented – as opposed to merely wishful – thinking that now characterizes this field of research.

Participants in the European Space Agency's Darwin and NASA's Terrestrial Planet Finder programs will do a lot of this thinking in November at a three-day meeting in Pasadena, Calif. These coordinated projects aim to orbit several observing satellites each. Data from these satellites will be combined and analyzed to lay bare the optical and infrared signatures of Earth-like planets around other stars. Given continued funding, data could begin to pour in by the middle of the next decade.

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The prospects of finding Earth-like planets look good. Astronomers have already located some 200 larger planets. Most of these are giants and probably not capable of supporting life. Some orbit very close to their parent star – hardly an analogue of our own solar system. Astronomers had thought those planets probably formed farther out and migrated inward, sweeping up any would-be Earth-like bodies in the process. But Sean Raymond at the University of Colorado in Boulder says that research he and his colleagues published Sept. 8 in the journal Nature shows that "these early ideas were probably wrong."

Their computer-based simulations show how a giant planet sweeping inward to a close orbit could actually create favorable conditions for the formation of "a new class of ocean-covered and possibly habitable planets." The team concludes that about one-third of known alien planetary systems could harbor Earth-like habitable bodies. But Dr. Raymond cautions that "any life on these planets could be very different from ours."

That's the grain of salt with which one should take the signs-of-life study reported last week by Lisa Kaltenegger and Wesley Traub from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. By simulating how life changed Earth's atmosphere during six key evolutionary stages, they provide what Dr. Kaltenegger calls "signposts that astronomers will look for when examining truly Earth-like worlds."

Some 4 billion years ago, Earth was a water-covered planet with an atmosphere essentially of nitrogen, carbon dioxide, and hydrogen sulfide. Then the first bacteria appeared – organisms that pumped a lot of methane into the air. Blue-green algae replaced them. These photosynthetic organisms enriched the air with oxygen and cleared out much of the methane and carbon dioxide. Subsequent evolution of life changed the atmosphere until it attained its present, mainly oxygen and nitrogen, composition.

If astronomers find an alien "Earth" whose air is loaded with methane and has little oxygen, they can conclude that it's early days for that planet's life. If the air mixture is more like our own, life could be more advanced. Of course, things may be so alien that what happened on Earth does not apply. But at the very least, the study shows one way to think about the search for alien life.