The water you're sipping may taste fresh, but a lot of it is very, very old — older than the solar system itself.

"Half the water in our oceans is in fact older than the Sun," science columnist Torah Kachur said in an interview with CBC's Homestretch.

"What that means is a lot of the water that exists on our planet was made before our solar system and it came from a different part of our galaxy altogether."

The sun is 4.5 billion to 4.6 billion years old.

Scientists led by Ilse Cleeves, a theoretical astronomer from the University of Michigan, figured out the approximate age of the water on Earth by conducting computer simulations of two chemical processes that are known to form water in deep space.


CBC science columnist Torah Kachur

One is a reaction between individual hydrogen and oxygen atoms that takes place on tiny dust grains. The other is a hotter reaction that takes place in stars and involves molecules of hydrogen gas.

The colder reaction generates water that contains more deuterium, a heavier form of hydrogen.

"Think of it as hydrogen's fat cousin," Kachur said.

When Cleeves and her colleagues compared the results of their calculations with the amount of deuterium on Earth, they concluded that 30 to 50 per cent of Earth's water was made through the cold process. That means it must have been made before the hot sun was formed.

They published their results in the journal Science.

Scientists have evidence that water was carried to Earth by comets that bombarded the planet about 3.8 billion years ago.