Using a novel approach that may have application in the search for life on other worlds, a team of international astronomers using the European Space Observatory’s Very Large Telescope (VLT) has discovered evidence of life on the third planet from the Sun.
“We used a trick called earthshine observation to look at the Earth as if it were an exoplanet,” says Michael Sterzik (ESO), lead author of the paper scheduled to appear in the March 1 issue of the journal Nature.
“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.”
In the faint light of earthshine, the team looked for the signature combinations of biologically produced gases in Earth’s atmosphere — oxygen, ozone, methane and carbon dioxide – that indicate the presence of organic life.
During a waxing or waning crescent Moon, sunlight reflected from Earth illuminates the Moon's night side, making the entire disc faintly visible. By analyzing the reflected light, known as earthshine, astronomers have detected Earth's biosignature. Credit: ESO/B. Tafreshi/TWAN
Planetary biosignatures are difficult to detect with conventional methods, but the team employed a more sensitive technique known as spectropolarimetry, which looks at the polarisation of the light, as well as it’s brightness across the color spectrum.
Their observations revealed that Earth’s atmosphere is partly cloudy, that part of its surface is covered by oceans — and the presence of vegetation. Also detectable were changes in cloud cover and variations in vegetation as different parts of the Earth turned towards the Moon.
Co-author of the study Stefano Bagnulo (Armagh Observatory, Northern Ireland, UK) explains the advantages of using spectropolarimetry when searching for life on exoplanets:
“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. 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.”
“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.”
Techniques such as spectropolarimetry, in combination with the next generation of telescopes such as the E-ELT (the European Extremely Large Telescope), may finally answer the question of whether life exists elsewhere in the universe.
