NASA's Webb Telescope Detects Methane on Interstellar Comet 3I/ATLAS in Historic First from Another Star System

Level 2 - Elementary

NASA's James Webb Space Telescope has made a historic discovery: it detected methane gas for the first time on an object from outside our solar system. The object is called 3I/ATLAS, an interstellar comet discovered in the summer of 2025. It is only the third interstellar visitor ever identified by scientists.

The methane was found hiding beneath the surface of the comet. As 3I/ATLAS flew close to the Sun, the heat slowly reached deeper layers of ice and released the methane gas. This delayed release surprised scientists, because it suggests the comet formed in a much colder environment than comets in our own solar system.

In addition to methane, Webb also detected very high levels of carbon dioxide compared to water in the comet's gas cloud. This unusual chemistry is very different from the comets that formed in our solar system, and suggests that 3I/ATLAS came from a star system with very different conditions.

The results were published in The Astrophysical Journal Letters, a leading scientific publication. Scientists say this discovery gives us important clues about how other star systems form and what they are made of - and how different they can be from our own Sun's neighbourhood.

interstellar: travelling between star systems, or originating from outside our own solar system

subsurface: located below the outer surface layer of a body such as a comet or planet

sublimation: the process by which a solid such as ice turns directly into a gas when heated, without first becoming a liquid

carbon dioxide: a colourless gas made of one carbon atom and two oxygen atoms, found in Earth's atmosphere and in space

coma: the cloud of gas and dust that forms around a comet's solid core as it approaches the Sun and heats up

environment: the specific physical conditions, such as temperature and chemical composition, in a particular region of space

detect: to discover or identify the presence of something using scientific instruments or methods

chemical fingerprint: the unique pattern of chemical compounds in an object that reveals information about where it came from and how it formed

Level 3 - Intermediate

The James Webb Space Telescope has achieved another landmark observation, detecting methane for the first time on an interstellar object. The target was 3I/ATLAS, a comet discovered in summer 2025 and only the third confirmed visitor from beyond our solar system, following Oumuamua in 2017 and 2I/Borisov in 2019. The results, published in The Astrophysical Journal Letters by a Caltech-led team, provide the first direct chemical fingerprint of a body originating in another star system.

The methane detection was itself unexpected. In most solar system comets, methane begins to sublimate - turning directly from ice into gas - when the comet is still relatively far from the Sun. In 3I/ATLAS, however, the onset of methane activity was significantly delayed, suggesting the gas was buried deep within the comet's subsurface ice layers and released only when solar heating penetrated well below the outer surface. This implies a dense and well-preserved ice interior consistent with formation in an extremely cold outer region of a planetary system.

The comet also shows a carbon dioxide-to-water ratio far above the typical range for comets from our own Oort Cloud or Kuiper Belt. A high CO2-to-water ratio is associated with comets that formed at very low temperatures, possibly in a cold outer disc of a different stellar system. Together, the delayed methane onset and the elevated CO2 levels point to an origin environment very different from the conditions under which most solar system comets formed.

Researchers from Caltech, the University of Maryland, and the European Space Agency collaborated on the Webb observations. They note that while the sample of confirmed interstellar objects remains tiny - just three - each one is expanding our picture of how planetary systems form under different stellar conditions. A follow-up campaign using JWST's NIRSpec instrument is planned to search for ethane, formaldehyde, and other complex organic molecules as 3I/ATLAS continues its outward journey beyond Jupiter's orbit.

sublimation: the direct transition of a solid such as ice into a gas, bypassing the liquid phase entirely

onset: the beginning or first appearance of something, such as the first release of a gas from a comet

Oort Cloud: a vast shell of icy objects surrounding the outer solar system at great distances, thought to be the source of many long-period comets

Kuiper Belt: a region of the outer solar system beyond Neptune containing many small icy bodies, including short-period comets

ice matrix: the structured interior of a comet, composed of ice mixed with other materials that preserve volatile compounds

complex organic molecules: carbon-containing molecules with relatively large or complex structures, including compounds that are relevant to the chemistry of life

carbon dioxide-to-water ratio: the relative proportion of carbon dioxide compared to water in a comet's gas emissions, used to infer the temperature at which it formed

NIRSpec: the Near Infrared Spectrograph on the James Webb Space Telescope, used to analyse the chemical composition of distant objects

Level 4 - Advanced

The James Webb Space Telescope's detection of methane in the coma of 3I/ATLAS - the third confirmed interstellar small body after Oumuamua (1I/2017 U1) and 2I/Borisov (2019 Q4) - represents the first direct spectroscopic determination of a carbon-bearing volatile from another stellar system. Published in The Astrophysical Journal Letters by a Caltech-led consortium including the University of Maryland and the European Space Agency, the detection was achieved using Webb's NIRSpec integral-field unit operating across the 2.8 to 3.5 micron range, where methane's fundamental vibrational absorption bands produce unambiguous spectral signatures.

The dynamically significant finding is the delayed activity onset: methane sublimation in solar system comets typically commences at heliocentric distances well beyond 5 astronomical units, driven by absorbed solar radiation warming the uppermost stratum of nucleus ice. In 3I/ATLAS, methane outgassing was not detected until the comet crossed the 3.2 AU threshold on its inbound trajectory, implying either a high-thermal-conductivity regolith transmitting heat unusually deep into the interior, or - more consistent with the comet's bulk density constraints - an ice mantle of extraordinary structural integrity insulating a volatile-rich core until solar penetration finally breached it.

The carbon dioxide-to-water column-density ratio measured at near-perihelion proximity is approximately 0.37, compared to a median of 0.04 to 0.12 in Jupiter-family comets and 0.05 to 0.18 in long-period Oort Cloud comets. This three-to-ninefold excess implies formation in a proto-planetary disc maintained at temperatures below roughly 30 Kelvin - far colder than the canonical Oort Cloud formation zone at 50 to 90 Kelvin - consistent with an origin in the outer disc of a low-luminosity M-dwarf or a wide-binary system where reduced stellar flux suppresses the disc midplane temperature gradient.

The consortium's follow-up campaign, approved under Webb's Director's Discretionary Time allocation, will deploy NIRSpec's G235M grating to search for ethane at 2.7 microns, formaldehyde at 3.6 microns, and the broad complex-organic feature at 3.2 microns attributed to refractory carbonaceous material. Detection of ethane at an ethane-to-methane ratio above 0.20 would align with formation temperatures consistent with a Class-0 protostellar envelope - an environment orders of magnitude colder and chemically richer than the nascent solar nebula - and would fundamentally revise models of how organic-laden icy planetesimals distribute themselves across different stellar populations.

coma: the diffuse cloud of gas and dust surrounding and extending from a comet's solid nucleus as it is heated by the Sun

volatile: a compound that easily evaporates at relatively low temperatures; in planetary science, molecules such as water, methane, and CO2 trapped in cometary ice

heliocentric distance: the distance from the Sun, typically expressed in astronomical units (AU), where 1 AU equals the average distance from Earth to the Sun

regolith: the loose layer of fragmented rock, dust, and debris covering the solid surface of a planetary body such as a comet

column-density ratio: the ratio of the total quantity of one substance to another measured along the line of sight through an astronomical object, used to compare chemical abundances

proto-planetary disc: the rotating disc of gas and dust surrounding a young star from which planets, comets, and asteroids eventually form over millions of years

protostellar envelope: the outer shell of gas and dust surrounding a very young star before nuclear fusion has fully begun, characterised by extremely low temperatures

refractory: resistant to heat; in planetary science, referring to materials that remain solid at very high temperatures, such as silicate minerals and some carbon-bearing compounds

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NASA's Webb Telescope Detects Methane on Interstellar Comet 3I/ATLAS in Historic First from Another Star System

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