2025Natur.641..608X

Debris disks are exoplanetary systems that contain planets, minor bodies (asteroids, Kuiper belt objects, comets and so on) and micrometre-sized debris dust¹. Because water ice is the most common frozen volatile, it plays an essential role in the formation of planets^2,3 and minor bodies. Although water ice has been commonly found in Kuiper belt objects and comets in the Solar System⁴, no definitive evidence for water ice in debris disks has been obtained to date¹. Here we report the discovery of water ice in the HD 181327 debris disk using the near-infrared spectrograph onboard the James Webb Space Telescope. We detected the solid-state broad absorption feature of water ice at 3 µm including a distinct Fresnel peak at 3.1 µm, which is indicative of large, crystalline water-ice particles. Gradients in the water-ice feature as a function of stellocentric distance reveal a dynamic environment in which water ice is destroyed and replenished. We estimated the water-ice mass fractions as ranging from 0.1% at approximately 85 au to 21% at approximately 113 au, indicating the presence of a water-ice reservoir in the HD 181327 disk beyond the snow line. The icy bodies that release water ice in HD 181327 are probably the extra-solar counterparts of water-ice-rich Kuiper belt objects in our Solar System.