- By Clara Sousa-Silva on November 19, 2019
Imagine for a moment that an alien astrophysicist on some distant but inhabited planet is trying to find out whether Earth is inhabited. If they were looking in the last 100 years, there would have been several unequivocal pieces of evidence: radio signals, television broadcasts, emissions of artificial chemicals such as chlorofluorocarbons, or CFCs.
But for the billions of years of life on Earth that preceded this, we were much less obvious. The alien astrophysicist could have detected oxygen in our atmosphere, which is intriguing but not a surefire sign of life. A combination of oxygen and methane would be much more promising, but both gases can be false positives, as they could be the result of geochemical or atmospheric processes.
The bigger problem is that aliens would have to know what to look for. They might well find oxygen here but, if they were not oxygen-reliant life forms, it might not occur to them that we are. To do their jobs thoroughly, these aliens would not be able to rest until they had detected all of the gases in our atmosphere, at different altitudes, latitudes and longitudes, and analyzed how each gas fits into the context of our biosphere.