Astronomers using NASA, ESA and CSA’s James Webb Space Telescope have measured the most deuterium-rich methane yet detected in the Solar System. The finding suggests that the interstellar object 3I/ATLAS formed in a cold, remote and metal-poor environment, potentially offering clues to the chemistry of planet formation around other stars.
“The recent discovery of comet-like objects, namely 2I/Borisov and 3I/ATLAS, passing through the Solar System in hyperbolic, gravity-free orbits opens new windows into the chemistry of planet formation around other stars by delivering their contents to our own planetary systems for analysis,” said lead author Dr. Nathan Ross of NASA’s Goddard Space Flight Center and colleagues.
“Mechanical and compositional studies of these mysterious objects may provide clues to their origins.”
Shortly after 3I/ATLAS was discovered, several studies used models of galactic dynamics and chemistry to suggest that it formed in a low-metallicity star system more than 3 billion years ago. That interpretation was based on the object’s radial orientation and high geocentric velocity, although distinguishing between origins in the thick and thin disks has been difficult, the researchers said.
Deuterium points to a cold formation environment
Astronomers used Webb to measure the ratio of deuterium to hydrogen (D/H) in methane from 3I/ATLAS. Deuterium is a heavier isotope of hydrogen, and the amount incorporated into molecules can provide important information about the conditions in which those molecules formed.
The measured methane D/H ratio is approximately 3.33%—about 14 times higher than the ratio measured in comet 67P/Churyumov-Gerasimenko by ESA’s Rosetta spacecraft. That comet is the only one in which deuterated methane has been reliably detected.
The value also exceeds every other methane measurement in the Solar System and is at least 1.4 times higher than direct measurements of other organic molecules in the Solar System.
The researchers used Webb’s NIRSpec integral field unit to observe 3I/ATLAS’s coma on December 22 and 23, 2025, when the object was approximately 2.4 AU from the Sun. Along with methane and its deuterated form, the observations detected methanol, ethane, formaldehyde, water and carbon monoxide.
Carbon monoxide was the most abundant of these compounds in the coma, with a production rate approximately 3.6 times that of water.
Comparisons with Solar System materials
“Besides Comet Churyumov-Gerasimenko, the Solar System’s methane D/H ratio has been measured for Venus, the giant planets, Titan, and the dwarf planets Eris and Makemake,” the scientists said.
“Organic D/H ratios have also been measured in interplanetary dust particles, carbonaceous meteorites, and hypercarbonaceous Antarctic micrometeorites.”
“The 3I/ATLAS value is at least 1.4 times higher than other direct measurements of organic molecules in the Solar System, the closest being D/H = 1.5% for organic radicals in the carbonaceous chondrite meteorite Orgueil.”
Methane D/H has almost never been directly measured outside the Solar System, so there are no clear standards for comparison. To address this limitation, the authors used the deuteration of another hydrocarbon, cyclopropenylidene, as a proxy.
They estimate that methane D/H ratios can reach approximately 4–7% or more in some star-forming regions and cold cores. This suggests that the value measured in 3I/ATLAS is consistent with conditions in the interstellar medium.
“It is unclear whether theoretical models of star formation and disk chemistry can reproduce the methane D/H ratio in the ice of 3I/ATLAS,” the researchers said.
“This should ideally be confirmed by new chemical models.”
“On the other hand, there is preliminary evidence that the interstellar/protostellar D/H ratio may match that of 3I/ATLAS.”
The findings were published in Nature Astronomy on October 23, 2026.
N. X. Ross et al. Enriched methane D/H ratio of interstellar object 3I/ATLAS. Nature Astronomy, published online October 23, 2026. doi: 10.1038/s41550-026-03002-5.
Source: www.sci.news