Searching for Atmospheric Biomarkers
If we were to look at the Earth from a distance, the spectrum of its atmosphere would look very different from that of Venus or Mars. Whilst Venus and Mars show strong evidence of having carbon dioxide in their atmospheres, Earth exhibits the tell-tale signs of ozone and water as well as some carbon dioxide.
Ozone is a compound of oxygen, so finding large amounts of ozone would mean that there were large amounts of oxygen. Some scientists believe that large amounts of oxygen can only be produced by life - such as plants and trees on land and tiny plants (phytoplankton) in the oceans. So if we discovered a planet with ozone in its spectrum we might expect there to be life on it. In fact, space telescopes are now being planned that look specifically for ozone in the atmospheres of distant planets.
Such an indication is called a biomarker because it marks out the possibility that life could exist.
Of course biomarker gases, such as ozone, only relate to life as we know it. We have to remember that extra-terrestrial life may not be what we expect and therefore may not require oxygen. Indeed, there is life at the bottom of our oceans that survives without oxygen.
The SETI programme is an international project employing over 100 scientists, educators and support staff who are dedicated to searching for signals from advanced civilisations inhabiting a distant planet.
In truth, we don't know what an alien signal will be like. The first extra-terrestrial civilization to make contact is likely to be far more technologically advanced than humans and we could scarcely imagine their thought processes, let alone their reasons and methods for communicating with us. All we can do is make some educated guesses based on our own knowledge and technology.
SETI's first assumption is that the signal will be received at or around the frequency of 1420 MHz. This is the spectral frequency of hydrogen, the most common element in the universe, and therefore may be a universal reference point for different intelligent species.
In order to detect strong signals at this wavelength, SETI uses radio telescopes around the world, such as the Allan Telescope Array (pictured) to scan the night sky. They have even targeted specific nearby stars in an attempt to detect periodic signals beamed in our direction.
To learn more about the SETI programme, click here.