Employing spectral data acquired by the Mid-Infrared Instrument (MIRI) aboard NASA/ESA/CSA James Webb Space Telescope, astrophysicists have achieved the first direct detection of methane within an interstellar object, designated 3I/ATLAS.
Hubble captured this image of 3I/ATLAS on July 21, 2025, when the comet was 446 million km (277 million miles) from Earth. Image credit: NASA / ESA / David Jewitt, UCLA / Joseph DePasquale, STScI.
“Interstellar objects (ISOs) represent planetesimals that originated around alien stars before being gravitationally expelled from their nascent systems through kinetic interactions,” articulated Caltech astronomer Matthew Belyakov and his collaborators.
“These ISOs, during their brief passage through our Solar System, furnish distinct insights into extrasolar populations of smaller celestial bodies and serve as a crucial benchmark for evaluating the commonalities and divergences in planetesimal formation mechanisms across the Milky Way.”
3I/ATLAS stands as the third verified interstellar entity unearthed, following 1I/’Oumuamua and 2I/Borisov. Its estimated nucleus spans a diameter of 2.6 kilometers (1.6 miles).
In stark contrast to 1I/’Oumuamua, which presented a quiescent demeanor, 3I/ATLAS exhibits an expansive coma.
“A focused endeavor has been undertaken to meticulously characterize the chemical composition of 3I/ATLAS’ coma,” the research team stated.
“Spectroscopic analyses conducted from ground-based observatories at visible wavelengths revealed the presence of gaseous cyanogen and elemental nickel. Concurrently, radio observations executed by the ALMA interferometer contributed methanol and hydrogen cyanide to the detected molecular inventory.”
“Investigations conducted in the near-infrared spectrum utilizing the Webb and SPHEREx space telescopes, prior to perihelion passage, unveiled fluorescence signatures indicative of water, carbon dioxide, and carbon monoxide.”
“Subsequent SPHEREx measurements obtained post-perihelion indicated a substantial acceleration in carbon monoxide emission and the emergence of an additional spectral feature within the 3.2–3.4 μm range, strongly suggesting the presence of organic compounds.”
“Further substantiation of 3I/ATLAS’ dynamic activity evolution includes a perceptible shift towards bluer visible hues and an apparent disparity between its pre- and post-perihelion water production trajectories.”
The top image displays 3I/ATLAS as observed by Webb’s MIRI instrument, with superimposed contours delineating the spatial distribution of various gases at the observation time. Water vapor extends considerably beyond the nucleus due to its release from icy grains within the coma, whereas carbon dioxide and methane are predominantly concentrated near the comet’s core. The lower image presents the spectral data, identifying the emission features of the gases detected by Webb emanating from the comet. Image credit: NASA / ESA / CSA / STScI / M. Belyakov, Caltech / I. Wong, STScI / A. Pagan, STScI.
These recent Webb observations were procured using the MIRI instrument on two distinct occasions as 3I/ATLAS egressed from our Solar System following its solar flyby.
The initial observational period spanned December 15–16, 2025, when the comet was approximately 329 million kilometers (205 million miles) from the Sun. A subsequent observation occurred on December 27, by which time it had receded to about 379 million kilometers (236 million miles).
“Methane is characterized by its high volatility, meaning it readily transitions from a solid ice state to a gaseous phase,” the research team elaborated.
“Its delayed manifestation within comet 3I/ATLAS suggests it was situated beneath the comet’s superficial layer, shielded from sublimation until the thermal influence of the comet’s proximate solar encounter penetrated deeper strata of the icy subsurface.”
“The ratio of methane to water detected is unexpectedly elevated, with few analogous occurrences identified within our own Solar System.”
It was previously established that 3I/ATLAS is an unusual object rich in carbon dioxide, a characteristic confirmed by the Webb telescope’s findings.
The comet continues to expel significantly more carbon dioxide in proportion to water when contrasted with typical comets originating from our Solar System.
Collectively, the abundances of methane and carbon dioxide strongly indicate an origin distinct from any formation process that transpired around our Sun.
“Furthermore, Webb recorded a precipitous reduction in gas production as comet 3I/ATLAS journeyed farther from the Sun, with water exhibiting the most pronounced decline,” the scientists reported.
“This behavior aligns with expectations for an object of this nature; as the comet experiences diminished solar heating, its surface cools, consequently reducing the rate of ice vaporization.”
A scholarly publication detailing these discoveries was disseminated on April 8, 2026, within the esteemed journal Astrophysical Journal Letters.
_____
Matthew Belyakov et al. 2026. The Volatile Inventory of 3I/ATLAS as Seen with JWST/MIRI. ApJL 1001, L11; doi: 10.3847/2041-8213/ae5700
