With no spacecraft capable of reaching planets beyond the solar system, scientists must rely on innovative techniques to search for signs of extraterrestrial life.
Now, a group of researchers is proposing a novel approach using the James Webb Space Telescope (JWST), a collaboration between NASA, the European Space Agency, and the Canadian Space Agency: searching for methyl halides.
These gases—composed of carbon, hydrogen, and a halogen such as chlorine—are primarily produced by bacteria, algae, fungi, and plants on Earth. Scientists find them particularly intriguing as potential biosignatures because they absorb infrared light, which aligns with JWST’s observational strengths.
Researchers believe these gases may be found in higher concentrations on planets with hydrogen-rich atmospheres, a category known as "Hycean" worlds—named from the combination of "hydrogen" and "ocean." These planets, thought to host vast seas, fall under the broader classification of sub-Neptunes, which are larger than Earth but smaller than Neptune. Their size makes them more accessible for atmospheric observations than smaller, rocky planets like Earth.
“One of the great benefits of looking for methyl halides is you could potentially find them in as few as 13 hours with James Webb. That is similar or lower, by a lot, to how much telescope time you’d need to find gases like oxygen or methane,” said Michaela Leung, a planetary scientist at the University of California, Riverside, in a statement. “Less time with the telescope means it’s less expensive.”
The study, published in Astrophysical Journal Letters, follows a controversial 2023 investigation in which another team used JWST to examine the atmosphere of exoplanet K2-18 b. That research suggested a weak, unconfirmed signal for dimethyl sulfide—an organic compound produced by phytoplankton in Earth's oceans. The claim sparked debate among scientists, with skeptics questioning the strength of the signal and the assumption that the planet was a water world with a hydrogen-rich atmosphere. The team based its conclusion on the detection of methane and carbon dioxide, as well as the absence of ammonia, aligning with predictions for Hycean planets.
Regardless of the controversy, the debate underscored the need for further research into which atmospheric gases JWST is best equipped to detect and which compounds serve as reliable indicators of biological activity. Oxygen, for instance, may be difficult to identify with current technology.
The UC Riverside team’s new study suggests JWST could detect methyl chloride, a specific type of methyl halide, on Hycean planets if present at concentrations as low as 10 parts per million—comparable to levels found in some regions of Earth. Their computer simulations indicate that the telescope could detect the compound in as few as 14 observations.
JWST employs a technique called transmission spectroscopy to study exoplanetary atmospheres. When a planet passes in front of its host star, its atmosphere filters the starlight, absorbing certain wavelengths. By analyzing the missing portions of the light spectrum, astronomers can determine which molecules are present in the planet’s atmosphere.
While Hycean planets might harbor microbial life, they would not be habitable for humans due to their extreme environments. However, if methyl halides were discovered in the atmospheres of multiple exoplanets, it could suggest that microbial life is far more common in the galaxy than previously thought.
“That would reshape our understanding of life’s distribution and the processes that lead to the origins of life,” Leung said.
With JWST continuously pushing the boundaries of space exploration, the search for extraterrestrial life is becoming more sophisticated. If scientists successfully detect these biological markers, it could mark a major breakthrough in understanding the potential for life beyond Earth.
