Astronomers have analyzed Earthshine-- sunlight that bounces off Earth to illuminate a darkened area of the Moon-- to see if that light could reveal life on Earth, and they found it could indeed. By picking apart the light's spectrum, they could identify the signatures of gases, like oxygen, that quickly combines with other elements. Therefore, finding free oxygen in an atmosphere implies it is constantly being replenished by something. That something is life. Closer analysis of the light also hinted at various landmasses and areas of vegetation on Earth's surface.
Astronomers plan to use techniques developed to analyze Earthshine to tease apart light bounced off an exoplanet and learn more about such worlds-- hopefully finding alien life by reading the signature it leaves in the atmosphere of its home world. Another good thing is that reflected light is polarized, while direct starlight is not. So, even though the light of a star would completely overwhelm the light reflected off one of its planets, astronomers focused on polarized light can concentrate on the planet.
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