Astronomers using Atacama Large Millimeter/submillimeter Array (ALMA) discovered doubly deuterated water (D2O), or “heavy water”, in the protoplanetary disk around V883 Orionisprotostar located At a distance of 1300 light years in the constellation Orion. This discovery shows that some of the water found in comets – and perhaps even on Earth – is older than the disk star itself, providing breakthrough insight into the history of water in our solar system.
This artist's impressions show the evolution of heavy water molecules as they were observed in giant molecular clouds, planet-forming disks, and comets—before they eventually may have made their way to Earth. Image credit: NSF/AUI/NRAO NSF/P. Vostin/B. Saxton.
Water may have been brought to Earth by comet and/or asteroid impacts, following primordial material left over from the protoplanetary disk where our solar system originated.
However, it is unclear whether the water ice on these bodies formed first, such as in the protoplanetary disk phase, or whether it is much older and originated from a parent molecular cloud.
“Our discovery demonstrates beyond doubt that water visible In the planet-forming disk around V883, Orion should be older than the central star and formed in the very early stages of star and planet formation,” said Dr Margo Leemker, an astronomer at the University of Milan.
“This represents a major breakthrough in understanding the path of water through planetary formation and how this water got into our solar system and possibly Earth through similar processes.”
Chemical analysis of heavy water shows that these water molecules survived the brutal formation of stars and planets, traveling billions of kilometers through space and time and ending up in planetary systems like ours.
Rather than being destroyed and reformed in the disk, most of this water is inherited from the earliest, coldest stages of star formation, a cosmic byproduct that may also be present on Earth today.
“Until now, we weren't sure whether most of the water in comets and planets formed fresh in young disks like V883 Orionis, or whether it was 'pristine' coming from ancient interstellar clouds,” said Dr. John Tobin, an astronomer at NSF's National Radio Astronomy Observatory.
“Detection of heavy water using sensitive isotopologue ratios (D2OH2O) proves the ancient heritage of water and provides the missing link between clouds, disks, comets and, ultimately, planets.”
“This discovery is the first direct evidence of the interstellar journey of water from clouds to the materials that form planetary systems—unchanged and intact.”
teams paper was published this week in the magazine Nature Astronomy.
_____
M. Lemker etc.. Pristine ices in a planet-forming disk discovered by heavy water. Nat Astronpublished online October 15, 2025; doi: 10.1038/s41550-025-02663-y