Ancient Water Predates Stars: Comet Origins Revealed

Astronomers have discovered ancient water molecules in a planet-forming disk, confirming that some of the water contained in comets may predate the birth of stars. The discovery provides groundbreaking clues to the origin of water in the solar system.

▲ This artist’s image shows the evolution of water molecules (H₂O, HDO, and D₂O). These molecules have been observed in giant molecular clouds, planet-forming disks and comets, and may eventually reach Earth. (Source: ALMA)

The research team used the Atacama Large Millimeter and Submillimeter Array (ALMA) to detect doubly deuterated water (D₂O, heavy water) in the planet-forming disk around the young star V883 Orionis (V883 Ori). Analysis of the chemical fingerprint of heavy water shows that these water molecules survived the violent processes of star and planet formation, traveled billions of kilometers of space and time, and eventually entered planetary systems like ours. This water is not destroyed and re-formed in the planetary disk, but comes from the earliest and coldest stages of star formation, a remnant of the universe that may also be present on Earth today.

Until now, it was uncertain whether most of the water in comets and planets was formed anew in young disks like V883 Ori, or whether it came from ancient interstellar molecular clouds, so-called pristine water. This time, the research team detected heavy water through the sensitive isotope ratio (D₂O/H₂O), confirming the ancient origin of water, and for the first time found the missing key to the continuous evolution from molecular clouds, planetary disks, comets to planets. This result is the first direct observational evidence that water is transported from interstellar molecular clouds all the way to planet-forming materials, and that water molecules are not destroyed or reorganized in the entire process.

▲ Heavy water column density ratios of the planet-forming disk of V883 Ori and other young objects NGC 1333 IRAS 2A, B335, L483 and comet 67P. The colored background and side histograms in the figure represent the theoretically expected distribution of water isotope ratios under the material inheritance scenario (inheritance) and the ice re-formation scenario (reset) before the birth of the star. Blue area: inheritance model, assuming ≲10% of H₂O ice is destroyed; red area: reset model, assuming ≳70% of H₂O ice is destroyed due to photodissociation and photodesorption in the core collapse model. (Source: Nature)

Water is fundamental to life and planetary habitability. Understanding the origins of water in planetary systems can help us speculate on the conditions under which life may have formed, both within our own solar system and elsewhere. The discovery shows that many young planets and even distant alien worlds may inherit water billions of years older than themselves, reminding us that life is inseparable from the ancient history of the universe.

(This article is reprinted with permission from the Taipei Planetarium; the first picture is a schematic diagram, source: ALMA)

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