How can we search for life on exoplanets? Step one: examine the Earth as if it were an exoplanet. That’s the idea behind a recent look at earthshine.

Researchers aimed what’s officially called the Very Large Telescope, housed at the European Southern Observatory, at the moon. They measured earthshine: sunlight that reflects off the Earth, hits the moon and bounces back.

The scientists are looking for hints of life in the light—such as evidence of the gases associated with organic life. Such gas signatures are generally tough to find, because any planetary light is swamped by the light of its host star.

But in this effort, the research team also took into account polarized light. Because light reflected off a planet is polarized. But light from the star is not.

The analysis of earthshine correctly revealed that the planet it came from has a partly cloudy atmosphere and is covered with oceans and some vegetation. The researchers could even ascertain changes in cloud cover and vegetation as the Earth turned. The research was published in the journal Nature. [Michael F. Sterzik, Stefano Bagnulo and Enric Palle, "Biosignatures as revealed by spectropolarimetry of Earthshine"]

Similar scrutiny of actual exoplanets may one day reveal the first good suspects for harboring life—via their light.

—Cynthia Graber

[The above text is a transcript of this podcast.]