Life's a Gas

Sara Seager, William Bains and Janusz Petkowski compile list of potential gases to guide search for life on exoplanets.

Sara Seager and her team is confident the scientific means by which we will learn if there is life beyond our planet will be through the study and future observation of biosignature gases in exoplanetary atmospheres. But which gases should scientists be looking for?

A new approach published in the journal Astrobiology this week seeks to maximize the chances of identifying planets orbiting nearby stars that support life, by creating a comprehensive list of go-to biosignature molecules. The article is available free for download on the Astrobiology website until June 5, 2016.

Although a few biosignature gases are prominent in Earth's atmospheric spectrum (familiar among them O2, CH4, N2O), life on Earth is known to produce thousands of different molecules and scientists theorize that some may be able to accumulate at similar or higher levels on exo-Earths (e.g., dimethyl sulfide and CH3Cl) depending on the exo-Earth ecology and surface and atmospheric chemistry.

To maximize the chance of recognizing signs of life, Seager et al. take the approach that all stable and potentially volatile molecules should initially be considered as viable biosignature gases. The team then presents its systematically constructed lists of volatile molecules in different categories up to molecular stuctures in which six non-H atoms occur -- a total of about 14,000 molecules. Of these about 2500 are CNOPSH compounds (compounds containing Carbon, Nitrogen, Oxgen, Phospherous, Sulphur or Hydrogen), one-fourth of them known to be produced by life on Earth.

"This work reminds me of Darwin's voyage aboard The Beagle, exploring the vast diversity of life by sailing around the world," says Nancy Y. Kiang, PhD, Senior Editor of Astrobiology and a scientist at NASA 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."

More about Seager and Petkowski's work can be found here.