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Alan Alda in Scientific American Frontiers

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Recipe for Life

Calling Card

Which planets might harbor life? Scientists can't say for sure, with only the Earth as an example. But the two main planetary ingredients for life seem to be liquid water and time.

Water boils off planets too near their stars, but freezes solid on those too far away. The "life zone" is the region around a star in which planets can maintain liquid water. The size and location of this zone depends on the size and temperature of the star. Hot stars have wider zones than their cooler counterparts. But the hotter the star, the shorter its life span. The second requirement for life rules out these short-lived stars.

Life-sustaining planets likely orbit stars similar to our sun.  

It took about a billion years before even the simplest life forms debuted on this planet. Another three billion years or so elapsed before intelligence evolved. Very hot stars, which can burn out after only one million years, aren't around long enough for life to arise on their surrounding planets.

Ruling out the hottest and the largest stars, as well as the smallest and the coolest, the search for life must focus on stars of average size and temperature. In other words, stars like our own sun.

Neighborhood Census

With the Terrestrial Planet Finder (TPF), NASA scientists may one day have a tool for detecting life-giving elements on faraway planets. Called "the most ambitious science mission ever attempted by NASA," by the National Academy of Sciences, this special telescope will use light to gather data about astronomical bodies. Among other things, the TPF will look for any Earth-sized planets within about 50 light years of our own. It will then determine which of these planets- if any- lie in the life zones of their parent stars, and detect if any of the "signposts of life," such as chlorophyll, are present in the atmosphere. NASA is scheduled to launch the TPF in 2011.

Called "the most ambitious science mission ever attempted by NASA," the TPF will use light to gather data about astronomical bodies.

Data from the TPF will help fill in many of the unknowns in Drake's Equation. But, according to Dr. Charles Beichman, Chief Scientist of the Origins Program, the success of the TPF depends only "on the first two terms of Drake's Equation," says Beichman. "SETI jumps right to the end."

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Photos: NASA

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