Potential indicators of life on other planets can be created in the laboratory

One way to understand the potential for life on distant planets—those in other solar systems and orbiting different stars—is to study a planet’s atmosphere. Telescope images often capture traces of gases that can indicate the presence of life and habitable planets. But the results of a new study by researchers at the University of Colorado Boulder challenge that idea: Scientists created a type of gas often thought of as an indicator of life in a chemistry lab where no organisms are present.

The article, published today in Letters from the Astrophysical JournalResearchers have discovered that a type of molecule that scientists typically consider a sign of life, called a biosignature, may not be as reliable an indicator of life as previously thought. The researchers created dimethyl sulfide, a type of organic sulfur compound often made by marine microbes, in a reaction chamber using light and gases found in the atmospheres of many planets.

The researchers said creating dimethyl sulfide in the lab is exciting, but their findings upend previous research. The work is led by Nate Reed, a visiting scholar at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, and Ellie Browne, a CIRES associate researcher and professor of chemistry.

“The sulfur molecules we make are considered indicators of life because they are produced by life on Earth,” Browne said. “But we made them in a lab without life. So it may not be a sign of life, but rather a sign of something hospitable to life.” Organic sulfur compounds may not be reliable biomarkers, but could instead serve as markers of metabolic potential, the study authors said.

In search of life

NASA’s James Webb Space Telescope was launched in 2009. One of its missions is to capture images of exoplanets, planets outside Earth’s solar system, to understand their different atmospheres. Part of the satellite’s mission is to ask whether these planets support life.

The new study looks at what happens in a planet’s atmosphere when gases react with light to form “organic haze and associated gases,” aerosol particles formed by 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 key findings from the study that we saw was dimethyl sulfide,” Reed said. “This is an exciting discovery because it has been measured in exoplanetary atmospheres and was previously thought to be a sign of life on this planet.”

To recreate planetary atmospheres in the lab, Reed and Browne, along with co-authors including CIRES deputy director Maggie Tolbert, mimicked atmospheres in which light reacts with gases. In the new study, they used UV light to transform methane and hydrogen sulfide molecules into reactive species, which produce organosulfur gases, the biosignatures observed from the James Webb Space Telescope.

Reed stressed that while the results are exciting, they are limited to one type of atmosphere. “There are a wide variety of atmospheres, and we only studied small differences in one of them. You can’t study every atmosphere that exists in the lab,” he said.

The researchers hope their study will inspire other fundamental laboratory studies of basic chemical reactions, particularly those involving sulfur. Sulfur is difficult to handle: it’s sticky, smelly, and toxic. But not studying sulfur reactions prevents scientists from fully understanding what these results mean for biosignatures.

“When we look for these biosignatures, we tend to want to make a big splash by saying we’ve detected signs of life,” Browne says. “The atmosphere is very good at making a whole bunch of different molecules, and we’ve found that just because they can be made in the lab doesn’t mean they’re not a source.”