Is it alive or is it merely an illusion? As scientists continue to hunt for life on alien worlds, they're trying to find out how to tell the difference between evidence of life and false positives. Now, scientists have found a new way to accomplish just that.

Powerful devices such as the James Webb Space Telescope, which is set for launch in 2018, may help astronomers look for life on a handful of faraway worlds by searching for evidence of oxygen in their atmospheres. This is known as a "biosignature." This is largely done with transit spectroscopy, which involves studying the spectral features of light visible through a planet's atmosphere when it transits or passes in front of its host star.

Oxygen on Earth is produced almost entirely by photosynthesis with the help of plants and algae. This means that Earth's biosignature is indeed evidence of life. However, this may not be universally true; some worlds can create oxygen without the presence of life. This is mostly common in the case of planets orbiting low-mass stars, which are smaller and dimmer than our sun, and are the most common in the universe.

"The potential discovery of life beyond our solar system is of such a huge magnitude and consequence, we really need to be sure we've got it right - that when we interpret the light from these exoplanets we know exactly what we're looking for, and what could fool us," said Edward Schwieterman, a doctoral student in astronomy and one of the researchers.

The researchers also found an indicator for abiotic oxygen resulting from starlight similarly breaking down atmospheric water, allowing hydrogen to escape and leaving vast quantities of oxygen behind. In these cases, oxygen molecules collide with each other frequently and produce short-lived pairs of oxygen molecules that become O4 molecules. This, in turn, produces a unique signature.

"Certain O4 features are potentially detectable in transit spectroscopy, and many more could be seen in reflected light," said Schwieterman. "Seeing a large O4 signature could tip you off that this atmosphere has far too much oxygen to be biologically produced. With these strategies in hand, we can more quickly move on to more promising targets that may have true oxygen biosignatures."

The findings could help researchers narrow down which planets to look at a bit more closely. As the hunt for alien life continues, scientists are finding more and more ways to slim down the list of planets to investigate.