New research challenges the assumption that specific molecules detected on other planets are a sign of life, showing that they can be synthesized under controlled laboratory conditions without any biological activity
Research reveals that certain biological signatures, such as dimethyl sulfide, once thought to be a sign of life on other planets, can be produced under laboratory conditions.
One way to explore the potential for life on distant planets – those orbiting stars other than our Sun – is to study their atmospheres. Telescopic images can reveal gases that hint at the presence of life and sedentary conditions. But recent research from the University of Colorado at Boulder challenges that idea by showing that a gas considered by many to be a biological signature of life can be produced synthetically in a lab without any living organisms.
The study found that one type of molecule that scientists usually consider a sign of life, or biological signature, may not be as strong a sign of life as previously thought. This compound, dimethyl sulfide, an organic sulfur compound usually produced by marine bacteria, was created using light and gases that are common in the atmospheres of many planets.
The researchers say it was exciting to create dimethyl sulfide in the lab, but their findings overturn previous studies. The research is led by Nate Reid and Eli Brown.
"The sulfur molecules we create are considered signs of life because they are produced by life on Earth," Brown said. "But we created them in a laboratory without life - so maybe they are not a sign of life, but a sign of something welcoming to life." Organosulfur compounds may not be strong biomarkers, but could instead serve as markers of metabolic potential, according to the study authors.
NASA's James Webb Space Telescope launches in 2021. One of its missions is to take pictures of extrasolar planets to understand different atmospheres. Part of the spacecraft's mission is to ask whether these planets support life.
The new study examines what happens in a planet's atmosphere when gases react with light to form "organic haze and related gases," aerosol particles formed through atmospheric chemistry. The authors focused on sulfur-containing organic molecules, including dimethyl sulfide, which are secondary metabolic products of living organisms on Earth.
"One of the big findings in the research that we've seen is dimethyl sulfate," Reed said. "It is interesting because it has been measured in the atmospheres of extrasolar planets, and was previously considered a sign of life living there."
To recreate the atmospheres of extrasolar planets in the laboratory, the team simulated atmospheres where light reacts with gases. In the new study, they used UV light to turn methane and hydrogen sulfide molecules into reactive species, producing organosulfur gases—the biological signatures seen in the web.
Reed noted that while the findings are exciting, they are limited to one type of atmosphere. "There is a wide variety of atmospheres, and we only examined small differences in one of them - it is impossible to study all the atmospheres that exist in the laboratory."
Looking to the future, the researchers hope that their research will inspire more fundamental laboratory studies that will examine basic chemical reactions, especially with sulfur. Sulfur is difficult to work with, but avoiding it will prevent scientists from fully understanding what these findings mean for biological signatures.
"When we look for these biological signatures," said Brown, "the tendency is to create a sensation: 'We discovered signs of life.'
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