James Webb Captures Methane on Interstellar Comet 3I/ATLAS

Jun 4, 2026

Soumyadeep Mukherjee

James Webb Space Telescope captures Methane on interstellar comet 3I/ATLAS cover

The James Webb Space Telescope has detected methane in the coma of interstellar comet 3I/ATLAS, adding a major new result to the growing field of interstellar object science. Using the Mid-Infrared Instrument (MIRI), JWST identified methane, water vapor, and carbon dioxide in material escaping from the comet’s nucleus as it moved through the inner Solar System. The observations provide one of the most detailed chemical studies ever performed on an object that formed around another star.

The results continue a trend established by earlier interstellar visitors. Both ʻOumuamua and Comet 2I/Borisov challenged existing ideas about small-body formation and evolution beyond the Solar System. Now, 3I/ATLAS has introduced another chemically complex object into that discussion. Unlike previous detections, however, JWST’s infrared sensitivity allowed researchers to study the distribution of gases within the comet’s coma in much greater detail.

Discovery of 3I/ATLAS and its interstellar origin

Astronomers first identified 3I/ATLAS through the Asteroid Terrestrial-impact Last Alert System, commonly called ATLAS. Early orbital calculations immediately drew attention because the comet followed a strongly hyperbolic trajectory. Such trajectories suggest that the object did not originate within the Solar System.

Unlike periodic comets that orbit the Sun repeatedly, interstellar objects enter the Solar System only once. Gravitational interactions in their original planetary systems likely ejected them into interstellar space long ago. Afterward, they drifted through the galaxy for millions or even billions of years before eventually passing near another star system.

As 3I/ATLAS approached the Sun, solar heating triggered increasing activity on the nucleus. Frozen volatile compounds sublimated into gas and dust, creating a bright coma surrounding the comet. This process exposed material that had remained trapped beneath the surface since the object formed around its parent star.

Ground-based observatories began monitoring the comet across multiple wavelengths. Soon afterward, astronomers scheduled JWST observations to examine the comet’s volatile chemistry in the infrared regime.

This is a Hubble Space Telescope image of the interstellar comet 3I/ATLAS. Hubble photographed the comet on July 21, 2025, when the comet was 277 million miles from Earth. Credit: NASA, ESA, David Jewitt (UCLA); Image Processing: Joseph DePasquale (STScI)

JWST’s MIRI instrument maps the comet’s chemistry

The James Webb Space Telescope has revolutionized infrared astronomy since it began scientific operations. Although much public attention focuses on distant galaxies and exoplanets, JWST has also become an exceptionally powerful observatory for Solar System science.

For comet research, infrared observations remain especially important because many volatile molecules produce strong spectral signatures at infrared wavelengths. Instruments like MIRI can separate these wavelengths and identify the chemical composition of escaping gases.

JWST observed 3I/ATLAS using MIRI spectroscopy and imaging. The data revealed strong signatures from water vapor and carbon dioxide, along with the unexpected methane detection. Scientists also examined how these gases are distributed around the nucleus.

The resulting observations showed that water vapor extended much farther outward into the coma, while methane and carbon dioxide remained concentrated closer to the central region. This difference provides insight into how various volatile compounds escape from the comet and respond to solar heating.

This image from the Mid-Infrared Instrument (MIRI) shows the interstellar comet in three different light wavelengths and illustrates where different gases were located at the time the comet was viewed. Credit: NASA, ESA, CSA, STScI, M. Belyakov (Caltech), I. Wong (STScI), Image Processing: A. Pagan (STScI)

Importance of Methane detection in a comet

Methane occupies an important role in cometary science because it responds strongly to thermal evolution. In active cometary environments, repeated stellar heating gradually removes methane from near-surface layers. As a result, methane-rich material often survives only in deeper and colder regions of a nucleus.

The methane detection on 3I/ATLAS suggests that the comet preserved rudimentary volatile reservoirs beneath its outer crust. Solar heating during its current passage through the Solar System likely exposed these deeper layers for the first time in a very long period.

Methane-rich icy bodies generally form in extremely cold regions where volatile compounds can condense and remain stable over long timescales. Scientists now suspect that 3I/ATLAS formed in a distant cold region around another star before gravitational interactions expelled it into interstellar space.

The observations may also help researchers compare chemical diversity between planetary systems. Some Solar System comets show methane depletion, while others retain substantial volatile inventories. JWST’s results indicate that interstellar comets may preserve chemical combinations not commonly observed in local comet populations. Future discoveries may eventually reveal whether the chemistry observed in 3I/ATLAS represents a common feature of interstellar comets or an unusual exception.

The NASA/ESA/CSA James Webb Space Telescope observed interstellar Comet 3I/ATLAS on 6 August 2025 using its Near-Infrared Spectrograph. Credit: NASA, ESA, CSA, M. Cordiner (NASA-GSFC)

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