The carbon monoxide-rich interstellar comet 2I/Borisov

Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our Solar System¹. Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles², organic material and prebiotic chemicals around their host system³. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions⁴. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide⁵. Here we report that the coma of 2I/Borisov contains substantially more CO than H₂O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner ('2.5 au) Solar System⁴. Our ultraviolet Hubble Space Telescope observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own.