Deuterium-enriched water ties planet-forming disks to comets and protostars

Water is a fundamental molecule in the star and planet formation process, essential for catalysing the growth of solid material and the formation of planetesimals within disks^1,2. However, the water snowline and the HDO:H₂O ratio within proto-planetary disks have not been well characterized because water only sublimates at roughly 160 K (ref. ³), meaning that most water is frozen out onto dust grains and that the water snowline radii are less than 10 AU (astronomical units)^4,5. The sun-like protostar V883 Ori (M_* = 1.3 M_⊙)⁶ is undergoing an accretion burst⁷, increasing its luminosity to roughly 200 L_⊙ (ref. ⁸), and previous observations suggested that its water snowline is 40-120 AU in radius^6,9,10. Here we report the direct detection of gas phase water (HDO and [Formula: see text]) from the disk of V883 Ori. We measure a midplane water snowline radius of approximately 80 AU, comparable to the scale of the Kuiper Belt, and detect water out to a radius of roughly 160 AU. We then measure the HDO:H₂O ratio of the disk to be (2.26 ± 0.63) × 10^-3. This ratio is comparable to those of protostellar envelopes and comets, and exceeds that of Earth's oceans by 3.1σ. We conclude that disks directly inherit water from the star-forming cloud and this water becomes incorporated into large icy bodies, such as comets, without substantial chemical alteration.