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Fall 2007 Vol. 7 No. 3

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Cassini spacecraft surveys Saturn's ring system, courtesy NASA/JPL/Space Science Institute Life in the Universe

Broadening the Search for Extraterrestrial Life

"No discovery that we can make in our exploration of the solar system would have greater impact on our view of our position in the cosmos, or be more inspiring, than the discovery of an alien life form," declares a recent National Research Council report. But what if in our search we weren't able to recognize alien life because we didn't know what we were looking at? Could alien life be entirely dissimilar to life on Earth?

To explore possible variations on how life could operate in the universe, the committee that wrote the report investigated what is known about the limits for life on Earth, as they may help define the limits for life elsewhere. Life can be found virtually everywhere on Earth when water and energy are both present, and the committee found that extreme conditions of water and temperature are the only situations that appear to limit or prevent the growth of organisms. Even if alien life forms differ radically from life on Earth, they may be confined by these same factors. Other conditions taken to extremes, including pH, pressure, radiation, and toxic metals, restrict the existence of many organisms, but as the discovery of a variety of microbes over the last few decades has revealed, certain organisms have the capacity to adapt to seemingly inhospitable conditions. The bacterium Deinococcus radiodurans can even survive radiation levels unknown in Earth's current natural environment.

©PhotodiscThe committee reviewed the basic requirements for terrestrial life and found several possible alternative biochemistries. For instance, DNA on Earth is composed of four particular chemical compounds called nucleotides, but lab experiments have shown that six or more nucleotides could be bound together in a structure that can encode genetic material and may support Darwinian evolution, a key requirement to life. The report notes that organisms with such DNA would not be perceived by standard detection systems designed to spot genetic material. Additional evidence also suggests that proteins, an essential constituent of all cells, need not be limited to the form they take on Earth.

Research also indicates that water might not be the only solvent organisms could use, and the committee concluded that there was "no compelling reason" to limit life in general to water. Under the right conditions, other liquids such as ammonia or formamide could serve the same role as water does in biochemical reactions.

Summing up the report's chief findings, committee chair John Baross said, "Our investigation made clear that life is possible in forms quite different than those on Earth. Advances throughout the last decade in biology and biochemistry show that the basic requirements for life might not be as concrete as we thought."

The findings of the report have obvious ramifications for the future search for alien life. NASA has so far focused on finding evidence of past or present water, but with the report's more comprehensive consideration of life, the search criteria should be broader, particularly to embrace the possibility that organisms might thrive with little or no water. Additionally, further research is needed in the area of life detection systems. NASA should also consider exploring areas of the solar system that might support alternative biochemistries earlier than currently planned, such as Saturn's moon Titan. "Nothing would be more tragic in the American exploration of space than to encounter alien life without recognizing it," the report emphasized.   -- Paul Jackson

The Limits of Organic Life in Planetary Systems. Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, Space Studies Board, Division on Engineering and Physical Sciences, and Board on Life Sciences, Division on Earth and Life Studies (2007, 116 pp.; ISBN 0-309-10484-X; available from the National Academies Press, tel. 1-800-624-6242; $30.25 plus $4.50 shipping for single copies).

The committee was chaired by John Baross, professor of oceanography at the University of Washington, Seattle. The study was funded by NASA.

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Copyright 2007 by the National Academy of Sciences