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ESA Science News
http://sci.esa.int

16 Jan 2003

Vital signs of life on distant worlds
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Detecting Earth-sized planets is hard enough but how does an
astrobiologist decide which of them are inhabited? Scientists are now 
working to understand what signals life might give off into space, so 
that when they do detect Earth-like planets they know what to look 
for.

Our radio and television broadcasts have been leaking into space since 
the 1930s, when the first powerful emitters were constructed. However, 
you can do things the other way around as well. The Search for 
Extraterrestrial Intelligence (SETI) has used radio telescopes to 
listen to the cosmos for similar signals.

Nowadays, astrobiologists are pinpointing more subtle signs that all 
life, not just intelligent life, might radiate into space. They call 
these telltale signatures 'biomarkers'.  When ESA's Darwin mission 
begins sending back data in the next decade, biomarkers will help 
indicate whether neighbouring planets are inhabited. If we only used 
radio waves as markers of life, this would exclude all forms of life 
that have not yet developed the means to emit radio waves. Life has 
been around for thousands of millions of years, but human beings have 
used radio waves for less than a century. Malcolm Fridlund, Project 
Scientist for ESA's Darwin mission says, "If other planets follow the 
Earth's pattern, it is much more likely that they will be inhabited by 
dinosaurs or even bacteria than by something that can count."

In the 1970s, the British scientist James Lovelock pointed out that, 
just by breathing, life affects the composition of the Earth's 
atmosphere. He suggested looking for similar effects as a way to 
search with telescopes for life on other planets. You can study the 
composition of an atmosphere by splitting a planet's light into a 
rainbow of colours. This 'spectrum' will contain dark lines made by 
various chemicals in the planet's atmosphere.

Darwin's strategy is to look for oxygen because oxygen is used by some 
life forms and produced as waste by others.  Scientists believe that 
without life, all free oxygen in a planet's atmosphere would disappear 
within just four million years, because it reacts so easily with other
chemicals. "The best estimates suggest that Darwin will be able to 
detect the build-up of oxygen caused within a few hundred million 
years of life's origin," says Fridlund.

Although Darwin will not detect oxygen directly, it will 'see' ozone, 
a form of oxygen. It will also see carbon dioxide, water, and, in 
certain cases, methane. Fridlund says, "The general consensus is that 
if we find ozone, liquid water, and carbon dioxide simultaneously, it 
will be a very strong indicator of life's presence."

The work will not stop once Darwin completes its survey of the nearest 
several thousand star-planet systems. Once it finds a living planet, 
the race will be on to understand the nature of its life forms. That 
means searching for more specific biomarkers. In future space 
missions, for example, scientists may use chlorophyll as a biomarker. 
This molecule allows plants and certain bacteria to use light as an 
energy source. "Finding the next generation of biomarkers is a very 
active field of research at the moment," says Fridlund.

He can see a future in which space telescopes look for intelligent 
civilisations by searching for industrial pollution in a planet's 
atmosphere. This may sound like science fiction but for now, oxygen, 
carbon dioxide, and water will be good enough as indicators of life.

For more information, please contact:

Dr Malcolm Fridlund
ESA - Darwin Project Scientist
ESTEC, The Netherlands
Tel: +31 (0)71 565 4768
E-mail: malcolm.fridlund{at}esa.int

ESA - Science Programme Communication Service
Tel: +31 (0)71 565 3273
E-mail: irina.bruckner{at}esa.int

USEFUL LINKS FOR THIS STORY

* More about oxygen, one of Earth's biomarkers
  http://spdext.estec.esa.nl/content/doc/47/31303_.htm
* More about Darwin
  http://sci.esa.int/darwin/

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