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:H 2 O 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 burst 7 , 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:H 2 O 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.