Science & Technology

Here's Why Oxygen on Exoplanets Doesn't Always Mean Life

New research debunks the conventional wisdom that oxygen and organic compounds in the atmosphere are strong evidence of alien life on a distant planet.

Those substances can wind up in exoplanet atmospheres for reasons that have nothing to do with biological processes, researchers learned by running laboratory simulations of atmospheric chemistry.

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"Our experiments produced oxygen and organic molecules that could serve as the building blocks of life in the lab, proving that the presence of both doesn't definitively indicate life," says first author Chao He, assistant research scientist in the earth and planetary sciences department at Johns Hopkins University. "Researchers need to more carefully consider how these molecules are produced."

Oxygen makes up 20 percent of Earth's atmosphere and is considered one of its most robust biosignature gases. But little is known about how different energy sources initiate chemical reactions and how those reactions create biosignatures like oxygen in the search for life beyond Earth's solar system.

Other researchers have run photochemical computer models to predict what atmospheres might create on exoplanets — those circling stars other than our own — but He believes this is the first time scientists conducted lab simulations.

A simulated carbon dioxide-rich planetary atmosphere exposed to a plasma discharge in a lab experiment.

9 Mixtures

Researchers performed the simulation experiments in a specially designed Planetary HAZE (PHAZER) chamber in the lab of coauthor Sarah Hörst, assistant professor of Earth and planetary sciences.

They tested nine different gas mixtures, consistent with predictions for super-Earth and mini-Neptune type exoplanet atmospheres. These kinds of exoplanets are the most abundant type of planet in our Milky Way galaxy.

Chao He explaining how the study's PHAZER setup works.

Each mixture had a specific composition of gases such as carbon dioxide, water, ammonia, and methane, and each was heated at temperatures ranging from about 80 to 700 degrees Fahrenheit.

He and colleagues allowed each gas mixture to flow into the PHAZER setup and then exposed the mixture to one of two types of energy, meant to mimic energy that triggers chemical reactions in planetary atmospheres: plasma from an alternating current glow discharge or light from an ultraviolet lamp.

False Positives

Plasma, an energy source stronger than UV light, can simulate electrical activities like lightning and/or energetic particles, and UV light is the main driver of chemical reactions in planetary atmospheres such as those on Earth, Saturn, and Pluto.

After running the experiments continuously for three days, corresponding to the amount of time gas would be exposed to energy sources in space, the researchers measured and identified resulting gasses with a mass spectrometer, an instrument that sorts chemical substances by their mass to charge ratio.

They found multiple scenarios that produced both oxygen and organic molecules that could build sugars and amino acids — raw materials for which life could begin — such as formaldehyde and hydrogen cyanide.

"People used to suggest that oxygen and organics being present together indicates life, but we produced them abiotically in multiple simulations," He says. "This suggests that even the co-presence of commonly accepted biosignatures could be a false positive for life."

The paper appears in ACS Earth and Space Chemistry. NASA's Exoplanets Research Program and the Morton K. and Jane Blaustein Foundation funded the work.

This article is republished from Futurity under a Creative Commons license. Read the original article.

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Learn more about the search for extraterrestrial life in "The Eerie Silence: Renewing Our Search for Alien Intelligence" by physicist and astrobiologist Paul Davies. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Chanapa Tantibanchachai of Johns Hopkins University January 18, 2019

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