According to a team of scientists led by Prof. Sara Seager from the Massachusetts Institute of Technology, gases emitted by alien life forms on habitable extrasolar planets could be detected remotely by telescopes, but these gases might have quite different compositions from those in the atmosphere of our planet.

“Thousands of exoplanets are known to orbit nearby stars. Plans for the next generation of space-based and ground-based telescopes are fueling the anticipation that a precious few habitable planets can be identified in the coming decade,” Prof. Seager and her colleagues explained.

“Even more highly anticipated is the chance to find signs of life on these habitable planets by way of biosignature gases.”

The team proposes that all stable and potential volatile molecules should be considered as possible biosignature gases.

In their paper published online in the journal Astrobiology, Prof. Seager and co-authors laid the groundwork for identifying such gases by conducting a massive search for molecules with six or fewer non-hydrogen atoms.

“To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases,” they said.

“We present a new approach to the subject of biosignature gases by systematically constructing lists of volatile molecules in different categories.”

“An exhaustive list up to six non-H atoms is presented, totaling about 14,000 molecules. About 2,500 of these are CNOPSH (C – carbon, N – nitrogen, O – oxygen, P – phosphorus, S – sulfur, and H – hydrogen) compounds.”

The team also describes how this list can help enhance our understanding of the limits of biochemistry on Earth.

“This work reminds me of Darwin’s voyage aboard The Beagle, exploring the vast diversity of life by sailing around the world,” said Dr. Nancy Kiang, Senior Editor of the journal Astrobiology and a scientist at NASA’s Goddard Institute for Space Studies.

“In the search for life beyond our planet, we are currently at a similarly exciting, early but rapidly evolving stage of exploration as the discovery of exoplanets accelerates.”

“Instead of netting strange creatures from the bottom of the sea, the authors here have searched and found thousands of curious, potentially biogenic gas molecules.”

“These will inspire a new body of research into identifying also larger molecules, investigating their origin and fate here, and their potential expression on exoplanets as signs of life.”

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Seager S. et al. Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry. Astrobiology, published online April 20, 2016; doi: 10.1089/ast.2015.1404