Scientists Created a List of Gases to Search for Life on Exoplanets

May 9, 2016 | Joanne Kennell

Artist's impression of life on other planets.
Photo credit: NASA

These molecules could be emitted by alien life-forms.

Scientists from MIT have developed a comprehensive list of molecules that could be present in the atmospheres of exoplanets in order to maximize our chances of identifying nearby star-orbiting worlds that support life.

These types of molecules are known as biosignature gases. If they are being emitted, or exhaled, by exoplanetary life-forms, they could be detected remotely by our next-generation space- and ground-based telescopes. However, these gases may have a different composition from those found in Earth’s atmosphere.

Although life on Earth produces thousands of biosignature gases, only a few, such as oxygen, methane, and nitrous oxide, are prominent in Earth’s atmosphere. However, there are other biosignatures that may be produced or accumulated to higher levels on exo-Earths, depending on the planet’s ecology, and surface and atmospheric chemistry.

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What gases should we be looking for?

The researchers propose that all stable and potentially volatile molecules should be considered as possible biosignature gases, and not just the molecules produced by life on Earth.

They constructed a list of non-hydrogen atoms that are stable (in the presence of water) and volatile (more likely to be in gas form in a planetary atmosphere) at standard temperature and pressure (normal conditions in the atmosphere at sea level).

The list totals around 14,000 gases. 2,500 are composed of the six biogenic elements (molecules that only contain the elements carbon, nitrogen, oxygen, phosphorus, sulfur, and hydrogen), 900 are inorganics, and roughly 11,000 are halogenated (contain fluorine, chlorine, bromine, iodine, and astatine) compounds.

Of this comprehensive list, only one-quarter (622) of the molecules containing the biogenic elements are known to be produced by life on Earth, and very few inorganics and halogen compounds are generated.

The purpose of this list is twofold. First, it can be used for the search of biosignature gases in exoplanetary atmospheres. Since the ecology and the evolution of other planets may be very different from Earth, it is important to consider and identify all the molecules that can accumulate in different types of hypothesized exoplanetary atmospheres.

Second, the list can be used to understand our own terrestrial biochemistry. Given that 99.999 percent of microbial species and 86 percent of all species on Earth are still undiscovered, there are probably plenty more chemicals — in the form of gases produced by life on Earth — left to be uncovered.

The paper has been published in the journal Astrobiology, and is available free for download on the Astrobiology website until June 5, 2016.

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