By studying Earth's Moon using The ESO's Very Large Telescope in Chile (below), an international team of astronomers have used a novel approach that may lead to future discoveries of life elsewhere in the Universe. Using a technique called earthshine observation to look at the Earth as if it were an exoplanet. This method establishes the Earth as a benchmark for the future search for life on planets beyond our Solar System.
When the Moon appears as a thin crescent in the twilight skies of Earth it is often possible to see that the rest of the disc is also faintly glowing. This phenomenon is called earthshine. It is due to sunlight reflecting off the Earth and illuminating the lunar surface.
After reflection from Earth the colors in the light are significantly changed. By observing earthshine astronomers can study the properties of light reflected from Earth as if it were an exoplanet and search for signs of life. The reflected light is also strongly polarised and studying the polarisation as well as the intensity at different colours allows for much more sensitive tests for the presence of life.
The fingerprints of life, or biosignatures, are hard to find with conventional methods, but the team has pioneered a new approach that is more sensitive. Rather than just looking at how bright the reflected light is in different colours, they also look at the polarisation of the light, an approach called spectropolarimetry. By applying this technique to earthshine observed with the VLT, the biosignatures in the reflected light from Earth show up very strongly.
"The light from a distant exoplanet is overwhelmed by the glare of the host star, so it's very difficult to analyse — a bit like trying to study a grain of dust beside a powerful light bulb," explains Co-author Stefano Bagnulo of Armagh Observatory, Northern Ireland. "But the light reflected by a planet is polarised, while the light from the host star is not. So polarimetric techniques help us to pick out the faint reflected light of an exoplanet from the dazzling starlight."
The team studied both the color and the degree of polarisation of light from the Earth after reflection from the Moon, as if the light was coming from an exoplanet. They managed to deduce that the Earth's atmosphere is partly cloudy, that part of its surface is covered by oceans and — crucially — that there is vegetation present. They could even detect changes in the cloud cover and amount of vegetation at different times as different parts of the Earth reflected light towards the Moon.
Finding life outside the Solar System depends on two things: whether this life exists in the first place, and having the technical capability to detect it," adds co-author Enric Palle (Instituto de Astrofisica de Canarias, Tenerife, Spain). "This work is an important step towards reaching that capability."
"Spectropolarimetry may ultimately tell us if simple plant life — based on photosynthetic processes — has emerged elsewhere in the Universe," concludes Sterzik. "But we are certainly not looking for little green men or evidence of intelligent life."
The next generation of telescopes, such as the E-ELT (the European Extremely Large Telescope), may well be able to bring us the extraordinary news that the Earth is not alone as a bearer of life in the vastness of space.
The Daily Galaxy via The ESO