One of these days we’re going to have a new generation of telescopes, some in space and some on the Earth, that can analyze the atmosphere of a terrestrial world around another star. It’s not enough to find individual gases like oxygen and ozone, carbon dioxide or methane. Any of these can occur naturally without ramifications for life. But finding all of these gases in the same atmosphere is telling, because without life to replenish them, some would disappear. Getting the data is going to be hard, which is why new work using the European Southern Observatory’s Very Large Telescope is so interesting.

The work involves ‘Earthshine,’ the reflection of sunlight off the Earth that is in turn reflected off the surface of the Moon. It’s faint, to be sure, but Earthshine is visible in a crescent Moon when the light of the entire lunar disc is visible although only the crescent is brightly lit. Michael Sterzik (ESO) and team have used Earthshine to analyze our own planet’s biosignature, and the results are encouraging. The researchers could deduce from the reflection not only that part of Earth’s surface was covered with ocean, but also that vegetation was present, and both cloud cover and vegetation varied with the rotation of the Earth.

The key is to look not only at brightness variations but at how the light is polarized. This approach, called spectropolarimetry, turns out to be extremely sensitive to biosignatures in reflected light, as co-author Stefano Bagnulo (Armagh Observatory, Northern Ireland) points out:

“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.”

Polarization tells us more than how bright a given object appears by revealing as well the orientation of the electric and magnetic fields that make it up. Think of the polarized light reflected off a wet road, which polarized sunglasses can reduce by suppressing part of the light (those of us with sensitive eyes rejoice in this fact). The polarized lenses pass only light whose electric vector is in a certain direction. Now we know that the direction of oscillation of the electromagnetic waves we’re studying can be a factor in exoplanet research, not only showing the presence of life but allowing us to separate a planet’s light from that of its host star.

Image: A table from the paper revealing strong biosignatures through spectropolarimetry. Credit: Michael Sterzik/ESO.

The team used the FOcal Reducer/low-dispersion Spectrograph (FORS) mounted at the Very Large Telescope in Chile to measure the linear polarization spectra of Earthshine, comparing its data to models for Earth-like extrasolar planets and also to data from the space-based POLDER (POLarization and Directionality of the Earth’s Reflectances) instrument, for periods in April and June of 2011. While the results are impressive, they may be most significant in helping us tune up our tools. The paper concludes “Improved vector radiative transfer models with more realistic cloud and surface treatment are necessary to fully account for the observed spectra.”

The paper is Sterzik et al., “”Biosignatures as Revealed by Spectropolarimetry of Earthshine,” Nature 483 (01 March 2012), pp. 64–66.