Earthshine holds clues to exoplanet aliens

IS IT possible to tell whether a planet hosts life just from its glow? A new analysis of Earthshine, sunlight reflected off Earth then bounced back by the moon, suggests this is a viable way to seek life on exoplanets.

Life co-exists with certain chemicals that leave their imprint on the light Earth reflects, while plants reflect light differently to rocks. The trouble is that exoplanets are too faint compared with their host stars for such distinctions to be detected.

So Michael Sterzik of the European Southern Observatory in Santiago, Chile, and colleagues used a spectrograph mounted on the Very Large Telescope to examine polarised Earthshine, its light waves aligned in one plane. The reflection of light off a planet's surface and passage through the atmosphere cause it to become polarised, making it visible amid the glare of unpolarised starlight.

The team found light signatures of oxygen, ozone and water, as well as an increase in reflected wavelengths characteristic of vegetation (Nature, DOI: 10.1038/nature10778). Future telescopes such as the European Extremely Large Telescope could look for these signs in exoplanet-shine.

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