Up to half of Earth's water is older than the sun -- and came here from space

Much of the water on Earth -- an estimated 30 to 50 percent -- came from space and is actually older than the sun, says a team of scientists from the University of Michigan at Ann Arbor.

New Scientist reported Friday that the discovery sheds light not only on how our planet and sun were formed, but also gives scientists clues about what makes up planets and stars outside our solar system. The team's results suggest that wet -- and therefore life-sustaining -- planets may be more common than previously thought.

"Water is an essential ingredient that pretty much all known forms of life on Earth need to flourish," said University of Michigan scientist Ilsedore Cleeves to New Scientist. "To understand where water came from tells us a little bit about how common life is in the universe."

Researchers challenged the prevailing theory about how our solar system was formed and at what point water entered the picture. It was previously believed that there was water in the dust cloud that swirled together to form our sun, but that the heat and radiation of the nascent star burned it all away.

As the sun formed, a disc of dust, ice and other cosmic debris formed a cloud around the new star, which is called the "protoplanetary disc." Over time, said the old theory, water formed as the planets solidified and fell into their orbits around the sun.

The research team at Ann Arbor studied heavy water -- formerly known as "deuterium oxide" -- a compound like water, but which has a higher amount of the hydrogen isotope deuterium. Heavy water is in all water on Earth, but occurs at a frequency of about 1 molecule to every 3,200 molecules of regular water.

Water from space, which exists as clouds of ice crystals floating between the stars, has a higher amount of deuterium. By examining the amounts of heavy water on Earth, the team began to compute how much of Earth's water came from interstellar ice.

Scientists used to believe that the early solar system was so violent and the radiation there so intense that incoming ice molecules would have been destroyed, torn apart into their component atoms.

"The question was whether that violent, hot process of star creation scrambled the chemicals and broke the molecules of this pre-solar gas's heavy water," Cleeves told the Washington Post. "But if it did, then water would have been formed locally in this protoplanetary disk."

Cleeves' team built a computer model of the early sun and found that the theory didn't add up. If all the interstellar ice disappeared, there just wasn't enough available oxygen to combine with hydrogen and make water.

"The punchline here is that we simulated heavy water formation as it would have occurred in the disk, and it was almost negligible," Cleeves told the Post.

If interstellar water can survive stars' formation, the team believes, then its existence on other planets is much more likely. Water, of course, is one of the key factors necessary for the formation of life as we know it.

University of Chicago planetologist Fred Ciesla told New Scientist that water isn't the only component for life that is floating between the stars. Other organic compounds, he said, could potentially survive these star-births and combine one on of the bodies that form from the stars' protoplanetary discs.

"It provides the opportunity for organic materials, and the things that are important to the formation of life, to at least be accessible to all planets out there," Ciesla said. "Whether or not it forms into aliens and little green men and women is a whole other story."

The team's full paper is available by subscription in the journal Science.