Astronomers have found compelling evidence for life in the Universe – but only on Earth. Using a phenomenon known as Earthshine, where light from Earth is reflected off the surface of the Moon, researchers from the European Southern Observatory were able to view the Earth as if it were an exoplanet, and use the reflected light to look for tell tale signatures of life.
We can get a very good idea of the composition of something from the light it emits or reflects. Certain molecules leave their fingerprints in the spectrum of light, and our understanding of the chemistry of these molecules can give us a good idea as to what is going on. However, observing light from planets around other stars is challenging – any reflected or emitted light will be drowned out by the glare of the parent star.
One solution comes in the fact that while emitted light, such as that from a star, will arrive in any and all orientations, reflected light is polarised – it comes in just one orientation. By looking at both the spectrum and the polarity, in a process known as spectropolarimetry, astronomers can look for chemical signatures in just the light reflected off a planet.
In this case, they were looking for biomarkers – evidence of certain combinations of gases in certain ratios that can only exist in the presence of life, such as molecular oxygen and methane. These chemicals can exist on a sterile planet, but settle into a different equilibrium in the absence of living organisms. They also looked for a “Red Edge” – a change in reflectance of near infrared radiation that indicates the presence of vegetation.
Using just Earthshine light, the researchers were able to determine the fraction of Earth’s surface covered in seas, and that it has a cloudy atmosphere. Crucially, they could also detect the existence of vegetation, and could determine that it varied across the surface of the planet.
While the presence of life on Earth may not be news to us, it is proof that this technique works, and can be used in future to look for evidence of photosynthetic plant life elsewhere in the universe.
It is an interesting idea to make use of the fact that reflected light is polarised. I didn't understand how this would be applied to detect life on exoplanets though. I assume that they are not seeking a moon of an exoplanet (or another planet) to get the light 3rd hand (as it were) but just taking advantage of the light from the exoplanet's sun being polarised in being reflected from the exoplanet's surface. I can see that this would allow some filtering to be applied to remove much of the unpolarised light coming from its sun but I am surprised that the gain from this is sufficient to be useful; I suppose every little helps - are there any figures to show the advantage of such a method. A factor of two would be my guess but this light will still be highly swamped by the light from its sun.
Very clever technique. Someone used his or hers mind there. yor_on, Sun, 4th Mar 2012