How To Identify Life

...from a few light-years away.

 If we are looking for life on an exoplanet, What do we look for? Well, radio signals and TV broadcasts would be sure signs, but awfully unlikely. Seeing trees or animals moving on the surface is proof but far beyond the ability of any telescope we can build or imagine building*.


Instead, we hope to analyze light from the planet. From this light we can identify the chemicals in the atmosphere using spectral analysis. Oxygen (O₂) would be a good sign. It reacts so well that most atmospheres would use it all to make CO₂ or H₂O unless there was life to release it. Computer models now show that it is actually possible for an atmosphere to have oxygen and even ozone without ever having life. So even though oxygen is still a good sign. It is not proof. For solid proof, you would need to find oxygen, ozone, carbon dioxide, and methane with no carbon monoxide.

(Oct'15) How Space Telescopes Will Find Earth 2.0: progress in seeing an exoplanet. *

*Currently, even seeing an exoplanet is beyond our ability, let alone seeing things on the surface. When astronomers announce finding a new planet, they haven't seen it. They have seen indirect proof that it exists. We can just barely imagine building a telescope so powerful that it can see an exoplanet as a single dot.