Utilizing spectral data from the Mid-Infrared Instrument (MIRI) on the NASA/ESA/CSA James Webb Space Telescope, astronomers have successfully detected methane in the interstellar object 3I/ATLAS. This marks the first direct observation of methane in such an object.
Hubble captured this image of 3I/ATLAS on July 21, 2025, when the comet was 446 million kilometers (277 million miles) from Earth. Image credits: NASA/ESA/David Jewitt, UCLA/Joseph DePasquale, STScI.
“Interstellar objects (ISOs) are planetesimals that originate around distant stars and are subsequently ejected from their formation systems,” explained Matthew Belyakov, an astronomer at the California Institute of Technology.
“During its brief passage through our solar system, 3I/ATLAS provides a unique insight into a population of small extrasolar objects, serving as a valuable reference for understanding the processes of planetesimal formation throughout the galaxy.”
3I/ATLAS is now recognized as the third confirmed interstellar object, following 1I/’Oumuamua and 2I/Borisov, featuring an estimated core diameter of 2.6 km (1.6 miles).
Unlike 1I/’Oumuamua, which appeared inactive, 3I/ATLAS has persisted in a comatose state for some time.
“Concerted efforts are currently underway to analyze the chemical composition of the 3I/ATLAS coma,” the astronomers noted.
“Ground-based spectroscopy has identified gaseous cyanide and atomic nickel, while radio observations with ALMA have detected methanol and hydrogen cyanide in the molecular inventory.”
“Near-infrared space-based observations before perihelion with Webb and SPHEREx have revealed fluorescence signatures from water, carbon dioxide, and carbon monoxide.”
“Post-perihelion SPHEREx measurements indicated a notable increase in carbon monoxide production along with additional emission features in the 3.2-3.4 μm range, likely linked to organic material.”
“Further indicators of evolving activity in 3I/ATLAS include a bluish hue and noticeable asymmetry between pre-perihelion and post-perihelion water production trends.”
This image displays 3I/ATLAS, as captured by Webb’s MIRI instrument, with contour lines illustrating the presence of various gases. Water vapor, predominantly from comatose ice particles, extends beyond the core, while carbon dioxide and methane are concentrated closer to it. The spectrum below labels the signature gases escaping from the comet. Image credits: NASA/ESA/CSA/STScI/M. Belyakov, Caltech/I. Wong, STScI/A. Pagan, STScI.
The recent observations from Webb were conducted using the MIRI instrument on two separate occasions, capturing 3I/ATLAS as it orbited the Sun and subsequently retreated from the solar system.
The initial observation occurred between December 15 and 16, 2025, when the comet was approximately 329 million km (205 million miles) from the Sun. A second observation followed on December 27, when the comet had retreated to around 379 million km (236 million miles).
“Methane is highly volatile, transitioning from solid ice to gas with ease,” the researchers stated.
“The late emergence of methane in Comet 3I/ATLAS indicates that the substance is likely buried beneath a surface layer, shielded from sublimation until the comet’s proximity to the Sun warms the deeper icy layers.”
“The ratio of methane relative to water found is unexpectedly high and shares few parallels in our solar system.”
3I/ATLAS was already noted for its unusual carbon-rich composition, and Webb’s observations have confirmed it remains distinct.
This comet consistently exhibits significantly higher levels of carbon dioxide compared to water, in contrast to typical comets in our solar system.
The presence of methane and carbon dioxide suggests a different origin narrative than those formed around the Sun.
“Additionally, Webb’s observations revealed a rapid decrease in gas production as Comet 3I/ATLAS moved away from the Sun, with water showing the most considerable decline,” the scientists explained.
“Such behavior is expected for an object like this. As the comet receives less solar heat, its surface cools, resulting in diminished ice evaporation.”
A study detailing these findings is set to be published on April 8, 2026, in the Astrophysical Journal Letter.
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Matthew Belyakov et al. 2026. Volatile inventory of 3I/ATLAS as observed by JWST/MIRI. APJL 1001, L11; doi: 10.3847/2041-8213/ae5700
Source: www.sci.news
