“We used a trick called earthshine observation to look at the Earth as if it were an exoplanet,” said Michael Sterzik from ESO in Garching, Germany. “The Sun shines on the Earth, and this light is reflected back to the surface of the Moon. The lunar surface acts as a giant mirror and reflects the Earth’s light back to us — and this is what we have observed with the VLT.”
The astronomers analyze the faint earthshine light to look for indicators, such as certain combinations of gases in Earth’s atmosphere, that are the telltale signs of organic life. This method establishes Earth as a benchmark for the future search for life on planets beyond our solar system.
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 colors, they also look at the polarization 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 strongly.
“The light from a distant exoplanet is overwhelmed by the glare of the host star, so it’s very difficult to analyze — a bit like trying to study a grain of dust beside a powerful light bulb,” said Stefano Bagnulo from the Armagh Observatory in Northern Ireland. “But the light reflected by a planet is polarized, 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 polarization of light from Earth after reflection from the Moon as if the light were coming from an exoplanet. They managed to deduce that 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 Earth reflected light toward 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,” said Enric Palle from the Institute of Astrophysics of the Canaries, 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,” said 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 Earth is not alone as a bearer of life in the vastness of space.