While searching life in other solar systems far and beyond, scientists often consider that oxygen presence in planet atmosphere indicates the sign of life. Now a new study by the Johns Hopkins University recommends a reconsideration of that rule of thumb.
Reproducing in the lab the environments of planets beyond the nearby solar system, scientists effectively made both organic compounds and oxygen, missing of life. This study fills in as a wake-up call for analysts who propose the presence of oxygen and organics on universes is proof of life there.
In the search for life beyond Earth’s solar system, however, little is known about how different energy sources initiate chemical reactions and how those reactions can create biosignatures like oxygen. While other researchers have run photochemical models on computers to predict what exoplanet atmospheres might be able to create, no such simulations have before now been conducted in the lab.
In the lab, scientists performed the simulation experiments in a specially designed Planetary HAZE (PHAZER) chamber. They tested nine different gas mixtures, consistent with predictions for super-Earth and mini-Neptune type exoplanet atmospheres; such exoplanets are the most abundant type of planet in our Milky Way galaxy.
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.
Scientists then 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.
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.
Scientists found that 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.
Chao He, first author of the study said, “People used to suggest that oxygen and organics being present together indicates life, but we produced them abiotically in multiple simulations. This suggests that even the co-presence of commonly accepted biosignatures could be a false positive for life.”
This study published in the ACS Earth and Space Chemistry – was funded by the NASA Exoplanets. Research Program Grant NNX16AB45G. Chao, He received funding from the Morton K. and Jane Blaustein Foundation.