By observing the way the moon reflects light from the Earth, astronomers have established that there's life on our planet - not as pointless as it seems, as the technique could help find life elsewhere.
Using ESO’s Very Large Telescope, they've analysed the faint earthshine light to look for indicators of organic life, such as certain combinations of gases.
"We used a trick called earthshine observation to look at the Earth as if it were an exoplanet," says Michael Sterzik of ESO.
"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."
Such biosignatures aren't easy to find with conventional methods, but the team's new spectropolarimetry approach 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.
"The light reflected by a planet is polarised, while the light from the host star is not," says Stefano Bagnulo of the Armagh Observatory in Northern Ireland. "So polarimetric techniques help us to pick out the faint reflected light of an exoplanet from the dazzling starlight."
And, they say, the biosignatures in the reflected light from Earth show up very strongly, showing the worth of the new technique.
They researchers were able 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," says Enric Palle of the Instituto de Astrofisica de Canarias in Tenerife.
"This work is an important step towards reaching that capability."