James Webb telescope detects methane on interstellar comet for first time

Yesterday 12:00

By: Dakir Madiha

James Webb telescope detects methane on interstellar comet for first time

The James Webb Space Telescope has recorded the first mid infrared spectroscopic observations of an interstellar comet, identifying methane in 3I/ATLAS as it leaves the solar system. The detection provides a direct view of volatile compounds formed outside our planetary system and preserved over cosmic timescales. Scientists used the telescope’s MIRI instrument in December 2025 after the comet passed its closest point to the Sun, capturing fluorescence signatures from water, carbon dioxide, atomic nickel and methane.

The methane signal grew stronger as the comet moved farther from the Sun, indicating that its outer surface layer had begun to erode. That outer layer had likely been altered by long exposure to cosmic radiation during its journey through interstellar space. As it dissipated, deeper layers of ice that remained largely untouched became exposed. Researchers estimate the comet has traveled through the galaxy for at least one billion years, and its high speed limited the observation window to a short period, making the data especially valuable.

Chemical analysis shows that the ratios of carbon dioxide to water and methane to water are higher than those typically measured in comets from the solar system. These elevated ratios provide a chemical fingerprint of the environment in which 3I/ATLAS formed, pointing to conditions different from those around the early Sun. The findings offer rare insight into planetary formation processes in distant star systems.

Separate measurements from the European Space Agency mission Juice detected strong water vapor emissions shortly after the comet passed the Sun in November 2025. Instruments recorded an outflow of about 2,000 kilograms of water per second. Most of this vapor came from the side facing the Sun, and a significant portion originated from ice grains suspended in the surrounding coma rather than directly from the nucleus, indicating complex activity in the comet’s structure.

Further observations using the Atacama Large Millimeter Array measured the ratio of deuterium to hydrogen in the comet’s water. The values are 30 to 40 times higher than those found in Earth’s oceans and exceed all known comets in the solar system. This enrichment suggests formation in an extremely cold environment exposed to intense ultraviolet radiation from young stars. The isotopic signature confirms the comet’s origin in a distant planetary system before it was ejected into interstellar space.

The James Webb telescope is expected to conduct a final observation of 3I/ATLAS before it moves beyond detection range, having already passed Jupiter’s orbit and continuing its trajectory into deep space.