The James Webb Space Telescope has delivered the first detailed infrared view of interstellar comet 3I/ATLAS, the third confirmed object to enter the solar system from another star. Using its MIRI imager and NIRSpec spectrograph during an observing run on 6 Aug 2025, Webb measured the chemical make-up of the coma while the visitor was 3.32 au from the Sun. The data show a coma dominated by carbon dioxide, with a CO2-to-H2O mixing ratio of 8 : 1—one of the highest ever recorded and 6.1-sigma above the trend seen in solar-system comets. Astronomers say the unusual composition points either to a nucleus intrinsically rich in carbon dioxide or to formation near the CO2 ice line of its home star, in an environment unlike that of our own planetary system. Webb’s high-resolution images also revise the comet’s size sharply downward. The solid nucleus is now estimated at roughly 1.7 miles across, far smaller than early ground-based estimates that conflated nucleus and dust. Despite its modest scale, 3I/ATLAS is venting about 9.4 × 10¹⁴ molecules of CO2 each second and shows little of the water vapour or carbon monoxide usually seen in comets nearing the Sun. Discovered by the ATLAS survey on 1 July 2025, the object is currently about 298 million miles from Earth and will pass no closer than 223 million miles on 17 December before heading back into interstellar space. The publicly released Webb data, together with parallel observations by SPHEREx, Hubble and several ground observatories, give researchers an unprecedented chance to compare comet chemistry across planetary systems and to refine models of planet formation beyond the solar neighbourhood.