Our Solar System formed about 4.6 billion years ago when a giant cloud of gas and dust collapsed under gravity. But some of the water trapped inside that cloud had already formed in the cold interstellar environment before the Sun was born.
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As the cloud collapsed, much of this ancient water became incorporated into the material surrounding the newborn Sun. Some survived the violent process of planet formation and became part of the icy and mineral-rich material that eventually built planets, asteroids, and comets.⸻
Scientists can investigate this ancient water using its isotopic fingerprint. The ratio of ordinary hydrogen to deuterium—the heavier form of hydrogen—provides clues about where and under what conditions water formed.⸻
This doesn’t mean all of Earth’s water came from interstellar space. Water could have had several sources, including material already present in the Solar System and water delivered or redistributed by asteroids and comets. Researchers are still working to determine the exact contributions of each source
