Considering humans don't have a spacecraft capable of traveling to planets beyond the solar system, scientists have to get creative if they want to find clues of life existing light-years away.
Now a group of scientists is proposing a new way for astronomers to look for signs of aliens using the James Webb Space Telescope, a collaboration of NASA and the European and Canadian space agencies: look for methyl halides.
These gases — composed of carbon, hydrogen, and a halogen, like chlorine — are mostly made by bacteria, algae, fungi, and plants. The chemicals are particularly intriguing as a potential sign of life because they can be detected in infrared light, which is Webb's viewing specialty.
Furthermore, scientists think the gases may show up in higher concentrations around worlds with hydrogen-thick atmospheres, a type of planet Webb scientists are hunting for. These worlds, thought to have seas, are called "Hycean," combining the words "hydrogen" and "ocean." They would be among a class of planets called sub-Neptunes: smaller than Neptune but larger than Earth.
Their larger size is what makes them easier targets to observe than small rocky worlds 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 in Riverside, in a statement. "Less time with the telescope means it’s less expensive."
The paper, published in Astrophysical Journal Letters, follows a controversial study in 2023, wherein another team used Webb to study the planet K2-18 b's atmosphere and found a tenuous, unconfirmed chemical signal for dimethyl sulfide gas. That molecule is produced by phytoplankton, or microalgae, in Earth's waters, and because it's believed the substance is only produced by living things, the previous team suggested it could be an indication of life.
The report created a lot of hubbub among habitable world experts. Skeptics criticized the weakness of the signal and other claims in the research, such as the belief that the planet is a water world with a hydrogen-rich atmosphere. The team was convinced it was a Hycean world because Webb detected other molecules, such as methane and carbon dioxide, and no signs of ammonia, consistent with predictions for these planets.
But one thing the debate emphasized was the need for more research on what kinds of gases Webb could be well-suited to detect, as well as which ones are good bets for biological activity. Oxygen, for example, might be impossible to detect, due to current technological constraints.
The new paper from the UC Riverside team posits Webb could detect one particular methyl halide, methyl chloride, on Hycean planets. The amount needed for detection is about 10 parts per million, which is similar to what is found in some areas on Earth. Computer simulations showed the telescope could detect it in as few as 14 observations.
Webb conducts atmosphere studies using a technique called transmission spectroscopy. When planets cross in front of their host stars, starlight is filtered through their air. Molecules within the atmosphere absorb certain light wavelengths, or colors, so by splitting the star’s light into its basic parts — a rainbow — astronomers can detect which light segments are missing to discern the molecular makeup of an atmosphere.
One thing to note about the theorized Hycean worlds: Although scientists are curious if microbes could exist on such planets, humans would not be able to breathe in that environment. Still, if astronomers began to find methyl halides in several exoplanets' atmospheres, it could mean microbial life isn't rare in the galaxy.
"That would reshape our understanding of life’s distribution and the processes that lead to the origins of life," Leung said.
