Vital signs of life on distant worlds
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 tell-tale 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.
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."
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.
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.