“This is sooooooooo exciting. Could go away with more data but it’s exactly the kind of path we’d expect on the road [to] finding life,” wrote astrophysicist Adam Frank of the University of Rochester. His sentiment captures the cautious enthusiasm rippling through the scientific community after the James Webb Space Telescope (JWST) detected dimethyl sulfide (DMS) in the atmosphere of exoplanet K2-18 b. But as exciting as the discovery is, it has also sparked a heated controversy: does this chemical really equate to life?
Dimethyl sulfide, a molecule composed of one sulfur atom and two methyls, is a ubiquitous substance on Earth. It’s produced primarily by ocean phytoplankton and aids in the formation of clouds by emitting aerosols in the air. But its most notorious characteristic is its overpowering stench, usually compared “Any of these sulfur things are going to be super stinky that garlic, rotten egg smell,” said Eleanor Browne, a chemist at the University of Colorado Boulder, in an interview with Scientific American. But aside from its terrestrial infamy, DMS has long fascinated astrobiologists because it is being studied as a biosignature a chemical sign of life.
The JWST observation of K2-18 b, a planet that orbits a red dwarf star 124 light-years away in the constellation Leo, has presented the strongest evidence yet for DMS off Earth. K2-18 b lies in the so-called “Goldilocks Zone” where temperatures may be just right for liquid water to flourish. The world, as a “Hycean world” through the assumed hydrogen-rich atmosphere and world ocean, is already known to contain methane and carbon dioxide in its atmosphere. These and DMS detections have raised hopes for scientists such as Nikku Madhusudhan, an astrophysicist at Cambridge University, that K2-18 b may be “teeming with life.”
In a just-released paper in The Astrophysical Journal Letters, Madhusudhan and colleagues employed two other JWST instruments to verify the existence of DMS and a related molecule, dimethyl disulfide. The readings showed concentrations thousands of times higher than on our planet. “You need thousands of times of Earth’s concentrations to be able to explain the data,” Madhusudhan depicted at a presentation webcast on YouTube. These types of figures are what will increase from a warm life-giving ocean being present on the planet, he concluded.
There are doubts expressed by other researchers, however. DMS had also been present in non-biological samples, on Comet 67P/Churyumov-Gerasimenko within the European Space Agency’s Rosetta mission. Lab experiments have also demonstrated the possibility of DMS being created abiotically under specific conditions. For example, scientists have succeeded in producing DMS by simulating an exoplanet atmosphere and subjecting it to ultraviolet radiation. These results prove the contention that DMS is not an absolute biosignature. “There’s always going to be a way to make something abiotically,” said Browne, indicating the richness of planet chemistry.
Oxford University astronomer Chris Lintott also endorsed against leaping to conclusions, stating that if biology were creating DMS on K2-18 b, then other chemicals related to it would also be present. “[Dimethyl sulfide] should exist in a chemical network. If it’s produced by biology, it should break down, and the raw materials such as H₂S [hydrogen sulfide] used to make it should be visible in the spectrum, too. They aren’t,” he explained to Astronomy.
The statistical credibility of JWST’s results has also been questioned. Although detection of DMS has been at a 3-sigma level i.e., 0.3% chance the signal would be a coincidence it is still short of the 5-sigma, which is the standard in science. “With 16 to 24 additional hours of observing time, they think they can hit the 5-sigma mark,” Astronomy reported. But even that, MIT planetary scientist Sara Seager cautioned, “enthusiasm is outpacing evidence.” She cautioned against jumping the gun on making too much of the discovery, considering the multiple ways DMS might be produced without life.
Skeptical Thomases notwithstanding, the find is a breakthrough in the hunt for extraterrestrial life. The JWST’s capability to pick up on faint chemical signs in faraway atmospheres is a testament to the telescope’s record sensitivity. “This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations,” said Madhusudhan to Sky at Night Magazine. “The signal came through strong and clear.”
The implications of these results are not limited to K2-18 b. They show the complexity of biosignatures and that several lines of evidence are required to confirm the existence of life.
For the moment, the controversy surrounding DMS on K2-18 b is a reminder of the challenge and thrill of astrobiology. Whatever the molecule is a sign of, or an artifact of abiotic chemistry, its detection brings us one step closer to answering one of humanity’s most fundamental questions: Are we alone in the universe?
