An interesting proposal has been made in the last few days for a method of detecting life on exoplanets. It isn't yet a definitive test, which would be the holy grail of astrobiology, but it is an interesting new approach.

As I have written in past articles, over the past two decades scientists have managed to discover approximately 2000 planets outside of our own solar system, and many of these are believed to have the correct size and orbital radius to be habitable for life. Unfortunately because these planets are so far away from us, there do not yet exist methods of searching for life on them. It is possible that they are perfectly habitable but just don't happen to have anything living on them.

One suggestion that gets a lot of discussion is to search the spectrum of light passing through the planet's atmosphere and look for signatures of either molecular oxygen or ozone. White light passing through oxygen will scatter specific frequencies of light back to our telescopes, while absorbing other frequencies. If a telescope were able to detect the very faint light from the planetary atmosphere, it is possible that we could detect the presence of oxygen in this way.

Unfortunately planetary scientists have indicated that the presence of sulphur dioxide and other gasses would obscure this signal to the extent that many habitable planets will not show any sign of oxygen, even though it would exist in their atmospheres. But now another proposal has been made using oxygen dimers instead.

For those who don't remember chemistry class, a dimer is a bound state of two molecules that isn't a molecule itself. In the case of oxygen, this means that two oxygen molecules are weakly stuck together in a way that they can easily be broken apart. These dimers are also more sensitive to atmospheric pressure, so if they are detected it would give evidence that the planet has an atmosphere with the correct pressure to maintain liquid water - another requirement for life. Oxygen dimers absorb infrared light at two specific frequencies, which is in the range that could be detectable by future space based telescopes and which is less likely to be hidden by other gasses.

It is an interesting proposal, although far from being proven effective. But if it works, it may be an efficient way to seek out exoplanets with pressurized oxygen atmospheres that contain liquid water and possible life. And if that is true, it may be the means to find alien life for the first time in history.

Note: This is a re-posting of an article that was originally published on March 13, 2014, but which got removed by a technical glitch on the website.