The interstellar comet 3I/ATLAS released a surprising amount of methane during its flight through the solar system – significantly more than is typical for comets in the solar system under similar conditions. This is proven by new measurement data from the James Webb Telescope. They are the first detection of methane in an interstellar object, as astronomers report. Also unusual is the delayed outgassing of the methane – this provides exciting clues about the comet’s extrasolar past.
So far, astronomers have only discovered and observed three interstellar “visitors” to our solar system – the object 1I/’Oumuamua, the comet 2I/Borisov and, in 2025, the interstellar comet 3I/ATLAS. With a diameter of around 2.6 kilometers, this was the largest, fastest and probably oldest of the three extrasolar celestial bodies. It passed the closest point in its orbit to the sun on October 29, 2025 and has been on its way out of our solar system ever since.
“During their short flight through our solar system, such interstellar objects offer us insights into the population of small extrasolar celestial bodies,” explain Matthew Belyakov from the California Institute of Technology in Pasadena and his colleagues. “Therefore, there has been a concerted effort to characterize the chemistry of the 3I/ATLAS coma.” The gas and dust-rich shell around the comet nucleus is formed when volatile substances outgas from the nucleus due to heating. At 3I/ATLAS, astronomers have already detected water vapor, carbon monoxide (CO), unusually high levels of carbon dioxide (CO2), as well as methanol and hydrogen cyanide.
3I/ATLAS also releases methane
Now astronomers have identified another chemical component of the interstellar comet: methane (CH4). For their analyses, they targeted 3I/ATLAS on December 16 and 27, 2025 using the MIRI spectrometer of the James Webb Telescope. At that time, the interstellar comet was on its way out of the solar system again and was 329 or 379 million kilometers away from the sun.
The spectral data recorded in the mid-infrared showed several clear signatures of methane. “This is the first direct detection of methane on an interstellar object,” the astronomers write. During the first measurement on December 16th, 3I/ATLAS released 4.2 x 10>sup>26 methane molecules per second. This amount is significantly higher than previously measured for comets in the solar system, as Belyakov and his team report. However, during the second measurement two weeks later, the methane emissions were only half as high.
Unexpected behavior
The surprising thing, however, is that methane is a highly volatile gas and would therefore have had to emit a lot of gas as the comet approached its closest point to the sun and during this perihelion. But when Belyakov and his colleagues looked for the spectral signature of methane in observation data from the James Webb Telescope from August 25, 2025, it was present in 3I/ATLAS, but unexpectedly weak:
“The CH4 values from August are almost an order of magnitude lower than would be expected from our later measurements,” the team explains. Although the interstellar comet was closer to the Sun in August and its surface was therefore warmer, less methane gas was escaping from the comet’s nucleus at that time. “Methane is hypervolatile and has a significantly lower sublimation temperature than CO2,” explain the astronomers. “Therefore, methane ice on the surface of 3I/ATLAS should have been outgassing violently at this time.”
First warming already in the origin system?
How can this be explained? According to astronomers, this suggests that 3I/ATLAS was heated more strongly before – possibly in its home planetary system. As a result, a large part of the methane ice on the comet’s surface was already outgassing at that time. “The remaining reservoir of original methane ice therefore lies in deeper layers of the interstellar comet,” Belyakov and his colleagues speculate.
Because the heat from the sun’s passage only slowly penetrated into these deeper areas of the comet’s nucleus, this methane was released with a delay. “This behavior is also evident with carbon monoxide – which is even more volatile than methane,” report the astronomers. “The carbon monoxide production in the December measurements was 40 times higher than at the perihelion of 3I/ATLAS.” The new measurements thus provide another piece of the puzzle in the study of interstellar comets.
Source: Matthew Belyakov (California Institute of Technology, Pasadena) et al., The Astrophysical Journal Letters, 2026; doi: 10.3847/2041-8213/ae5700