Did you ever look up at the sky on a clear night during a crescent moon and see the faint outline of the "dark" side of the moon?

What you are actually seeing is the reflection of the Earth’s light on the surface of the moon. The part of the moon you are observing really isn’t the dark side, or you wouldn’t be able to see it. Scientists call this reflection “earthshine.” They are studying the characteristics of this faint light to see if it could someday be used to determine the atmospheric and surface compositions of Earthlike planets outside the solar system (exoplanets), and whether these distant planets could host life.

A group of scientists led by Michael Sterzik at the European Southern Observatory are using the Focal Reducer/Low-dispersion Spectrograph (FORS) mounted on the Very Large Telescope in Chile to study the characteristics of earthshine. These characteristics are being used to create theoretical models that could identify Earthlike exoplanets harboring life.

Previous techniques have used the light bouncing back from exoplanets to determine the makeup of their atmospheres. However, these observations need to be fine-tuned in order to determine whether life exists on these planets.

The new technique used by Sterzik uses a property of light called polarization. Polarization tells scientists not only how bright an object appears, but also measures the shifting rotation of electromagnetic waves. Light reflected by surfaces such as water, land, or vegetation is polarized. This polarized light can be used to determine the characteristics of a reflecting surface. In the case of the reflected light called earthshine, it tells us the properties of Earth’s atmosphere and surface, Sterzik explains.

So how will these observations be used to determine if life exists on exoplanets? That’s where biosignatures come in. A biosignature is any substance such as an element, isotope, molecule or phenomenon that provides scientific evidence of past or present life.

When scientists look through their telescopes, they do not expect to see intelligent forms of life. However, they can detect gases such as oxygen, ozone, methane and carbon dioxide. These gases can occur without the presence of life. However, an abundance of these gases existing individually alongside each other could indicate the existence of life. If life isn’t present, these gases would react and combine with one another.

Current techniques only provide a rough characterization of giant exoplanets using today’s instrumentation and telescopes. Detecting water and oxygen in exoplanets is currently out of reach, and new telescopes and sensors will be needed to directly observe the characteristics of an exoplanet’s atmosphere.

In the meantime, scientists will continue explore earthshine in order to fine-tune theoretical models that could one day be used to determine if life is present on other planets.