A study of light from the Earth that has bounced off the Moon could help astronomers in their search for life on distant planets. That is the claim of astronomers in Chile, the UK and Spain who have showed that faint light from the Earth contains strong signals of the biological processes that occur on our planet.

So far about 760 extrasolar planets – or exoplanets – have been discovered orbiting stars other than the Sun. The ultimate goal of many astronomers studying exoplanets is to determine whether some of them harbour life. Doing so will likely involve spectroscopic studies of the light that is absorbed and/or emitted from the exoplanet to look for large quantities of molecular oxygen and methane in the atmosphere that could be signatures of life. Astronomers would also be looking for a sharp change in the planet's reflectivity as a function of wavelength – which would occur if an exoplanet had vegetation similar to that on Earth.

Reducing glare

An important challenge in the search for such biosignatures is how to separate the relatively dim light from the exoplanet from the glare of its companion star. One promising solution involves taking advantage of the fact that light that has reflected from a planet is polarized, whereas light from a star is normally unpolarized. In principle, a technique called spectropolarimetry could be used to distinguish between starlight and light from an exoplanet.

Now, Michael Sterzik of the European Southern Observatory (ESO) in Santiago, Chile, Stefano Bagnulo of the Armagh Observatory in the UK and Enric Palle of the Institute of Astrophysics of the Canary Islands have used spectropolarimetry to study earthshine and have shown that the technique could be used to search for signs of life on exoplanets.

The team used the ESO's Very Large Telescope (VLT) in Chile to study earthshine light that was collected on two different days in 2011 – one in April and the other in June. The team focused on wavelengths of 500–900 nm, which corresponds to visible and near-infrared light. On both days they found that the polarization was greatest – about 10% – at short wavelengths and tailed off to about 4% at 900 nm.

Land versus sea

One interesting difference between the two observations is that the polarization in June was about 3% higher than that seen in April. According to the researchers, this could be because very different parts of the Earth were facing the Moon when the observations were made. In April light was coming from an area centred on the Atlantic Ocean containing parts of South America, Africa and Europe. In June the light came mostly from the Pacific Ocean, with much less land surface visible from the Moon.

To further study the earthshine, the trio fitted the polarization spectra to a smooth function and looked for deviations from this function. These deviations reveal spectroscopic features associated with the absorption or emission of light from molecules of biological interest such as oxygen or chlorophyll. In both spectra they found a narrow feature at about 760 nm that corresponds to molecular oxygen. Large quantities of this form of oxygen are only expected to be found on planets containing life-forms that perform some sort of photosynthesis to produce molecular oxygen – in their absence any molecular oxygen would quickly react and vanish from the atmosphere.

Catching the red edge

The team spotted another tell-tale sign of vegetation, the "red edge", in one of the spectra. This is a large and abrupt change in the absorption of light by plants that occurs at about 700 nm. At shorter wavelengths, chlorophyll absorbs very strongly and therefore plants reflect little light – above 700 nm chlorophyll does not absorb light, which means that leaves are able to reflect much more sunlight back into space. The red edge was prominent in the April data but very faint in June. This appears to agree with the fact that the April observation included much more landmass than June.

The team also looked at how the presence of cloud cover over oceans and vegetation affected the measurements, which suggests that spectropolarimetry could also be used to study clouds on distant planets.

"Finding life outside our solar system depends on two things: whether this life exists in the first place, and having the technical capability to detect it," explains Palle. "This work is an important step towards reaching that capability." Sterzik adds "Spectropolarimetry may ultimately tell us if simple plant life – based on photosynthetic processes – has emerged elsewhere in the universe."

Gas giants will be first

However, it could be some time before the technique is applied to Earth-like exoplanets. Upgrades to existing telescopes such as the SPHERE instrument on the VLT and the Gemini Planet Imager on the Gemini telescope should be able to make polarization measurements on Jupiter-like exoplanets in a few years. While this could provide significant insights into the atmospheres of these gas giants, the study of much fainter rocky Earth-like exoplanets will probably have to wait until NASA's planned New Worlds Mission space telescope is launched in 2019.

The research is described in Nature 483 64.