Some researchers believe that we must expand the search for extraterrestrial life outside of our solar system.
"Scientists who scan the atmosphere of exoplanets looking for gases capable of supporting life look for much more than just oxygen, methane, and other familiar biological attributes," said Sarah Seager and William Bein of the Massachusetts Institute of Technology in a review article published March 6 in the journal Advances.
“We know that there will not be a huge number of acceptable planets,” Seeger reported via e-mail. "We want to make sure that we don’t miss a single biological trait, but we need to think outside the box. Oxygen is a sign of life here on Earth, but what are the chances that it will be present on exoplanets?"
To date, scientists have discovered more than 1,800 alien planets, most of which are very different from the worlds in our solar system.
"It's amazing that the most common type of planets in our galaxy is the type of planets that vary in size from Earth to Neptune. These planets are not Earth-type planets or gas giants and for them we do not have a formation theory," wrote Seager and Bane.
The variety of exoplanets enhances the very real possibility that alien life may be completely different from earth life, even if they inhabit a world similar to ours. For example, what could be life on a planet in which the atmosphere is dominated by molecular hydrogen instead of nitrogen and oxygen?
"Although such planets have not yet been observed, the theory suggests their existence," wrote Seager and Baine.
Based on this logic, researchers advocate an open approach in order to first identify all viable biological traits through systematic, comprehensive study.
“The short-term goal is to understand which molecules can be biological traits in exoplanet atmospheres. A systematic table of chemicals capable of supporting life will give a starting point to predict which molecules are stable, volatile and which can be detected remotely using space telescopes,” - added Seager and Bane.
For such a complex project, most likely, it will take years. But scientists have already begun to probe the atmospheres of exoplanets, using such tools as the Very Southern Telescope telescope of the European Southern Observatory in Chile. In the near future, the search will be strengthened thanks to the launch of the NASA Transiting Exoplanet Survey Satellite telescope in 2017 and the James Webb space telescope in 2018. The first will have to discover a number of nearby rocky planets, and the second will explore their atmosphere.
Massive ground-based telescopes, such as the Giant Magellan Telescope, the Thirty Meter Telescope and the European Extremely Large Telescope, which have a light-emitting surface of 24 meters, 30 meters and 39 meters, respectively, should increase the chances of searching.
But Seager and many other experts say that a space telescope with a mirror from 10 to 12 m is needed to search for biological signs. Such a tool can potentially analyze a sufficient number of exoplanets to draw some conclusions.