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.

星际彗星直接从遥远的原行星盘提供挥发物样本。2I / Borisov是我们太阳系^1中发现的第一个特别活跃的星际彗星。彗星是恒星在其行星形成过程中存在于其原行星盘中的气体，冰和尘埃的浓缩样本，它们使我们了解化学成分和丰度如何随距中心恒星的距离而变化。它们的轨道迁移在其宿主系统³周围分布了挥发物²，有机物质和益生元化学物质。在我们的太阳系中，远距离观测到数百颗彗星，太空任务近距离研究了其中几颗彗星⁴。但是，对太阳系外彗星的了解仅限于从其他恒星周围的彗星区域的遥远而未解决的观测中可以收集到的信息，而只有一次检测到一氧化碳⁵。这里我们报道2I /鲍里索夫的彗差显着更多的包含CO除H ₂ O气体，具有至少173％的丰度，多于比任何彗星在所述内（<2.5 AU）光伏系统先前测量的高三倍⁴。我们对2I /鲍里索夫的紫外线哈勃太空望远镜观测首次发现了另一颗与我们的恒星大不相同的原行星的冰含量和化学成分。