The interstellar comet 3I/ATLAS is carrying methane unlike anything measured in our solar system. Using the James Webb Space Telescope, NASA scientists found its methane is packed with a heavy form of hydrogen, at levels about 13 times higher than the only other comet where anyone has managed to measure it.

Webb NIRSpec maps of water, carbon dioxide and carbon monoxide around interstellar comet 3I/ATLAS. Image: NASA, ESA, CSA, STScI, M. Cordiner / Wikimedia Commons, CC BY 4.0, modified