The hunt for signs of life on planets beyond our solar system should cast as wide a net as possible, some researchers stress.
Scientists scanning the atmospheres of exoplanets for gases produced by alien life should look for more than just oxygen, methane and the other familiar biosignatures that swirl about in Earth's air, Sara Seager and William Bain, both of MIT, wrote in a review article published on March 6 in the journal Science Advances.
"We know there will not be huge numbers of accessible planets," Seager told Space.com via email. "We want to make sure we do not miss any signatures, by trying our best to think outside the box. Oxygen is a great biosignature gas for Earth, but what are the chances it will be present on an exoplanet?" [10 Exoplanets That Could Host Alien Life]
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To date, scientists have discovered more than 1,800 alien planets, most of which are very different from the worlds in our solar system.
"A specific, astonishing finding is that the most common type of planet in our galaxy are those with sizes between those of Earth and Neptune - a new class of planet that is neither terrestrial nor giant and one without an accepted theory for its formation," Seager and Bain wrote.
The diversity of exoplanets reinforces the very real possibility that alien life may be quite different from life on Earth, even if it inhabits a rocky world like our own. For example, what might live on "exo-Earths" whose atmospheres are dominated by molecular hydrogen instead of nitrogen and oxygen, as Earth's is?
"Although not yet observed, such planets are theoretically anticipated," Seager and Bains wrote.
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Based on that reasoning, the researchers advocate an open-minded approach that would first identify "all viable biosignature gases, through a systematic, exhaustive study both from the view of molecules (there is no shortage) and of planetary environments and where the candidate biosignature gas molecules would accumulate and survive," they wrote.
"The near-term goal is to understand which molecules could be biosignature gases in atmospheres of exoplanets; a systematic table of chemicals made by life will give a starting point for predicting which molecules are stable, volatile and detectable remotely by space telescopes," Seager and Bains added.
Such a challenging project would likely take years to complete, Seager told Space.com. But researchers can spare the time because a systematic search for signs of life on alien worlds is probably at least a decade away, she and Bains wrote.