Looking for Evidence of Life on Earth

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.

仅使用地球反照光,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.