Most people are aware of the water cycle, which dictates that most water on Earth isn’t really created or destroyed, but cycled from one place to another. It would stand to reason, then, that some of the water we encounter is older than its freshness would indicate, but exactly how old? According to a study by the University of Michigan, as much as half the water found on Earth is older than the solar system itself.

Settling a debate regarding exactly when the water on Earth and the solar system was formed, researchers Ilse Cleeves and Ted Bergin determined that anywhere from 30 to 50% of the water came out of the ancient molecular cloud that birthed the disc of dust and gas that formed planets 4.6 billion years ago, and even the Sun itself. That would make the water about one million years older than the solar system that followed. To reach these findings, the researchers ran a chemistry simulation.

“Chemistry tells us that Earth received a contribution of water from some source that was very cold—only tens of degrees above absolute zero, while the sun being substantially hotter has erased this deuterium, or heavy water, fingerprint,” Bergin said.

The simulation examined the ratio of two different kinds of water – the kind we know, and a “heavier” variety. Today, comets and our oceans hold a higher ratio of heavy water than the Sun. The goal of the simulation was to let solar system development play out and see what ratios it arrived at.

“We let the chemistry evolve for a million years—the typical lifetime of a planet-forming disk—and we found that chemical processes in the disk were inefficient at making heavy water throughout the solar system,” Cleeves said. “What this implies is if the planetary disk didn’t make the water, it inherited it. Consequently, some fraction of the water in our solar system predates the sun.”

Given that astronomers are always on the lookout for evidence of water on other planets (and by extension, the possibility of life), the findings could be instructive as to where they might apply their efforts.

“The implications of these findings are pretty exciting,” Cleeves said. “If water formation had been a local process that occurs in individual stellar systems, the amount of water and other important chemical ingredients necessary for the formation of life might vary from system to system. But because some of the chemically rich ices from the molecular cloud are directly inherited, young planetary systems have access to these important ingredients.