Up to half the water on Earth is likely older than the solar system, raising the likelihood that life exists elsewhere in the galaxy, according to a study.
The research published in the journal Science on Thursday found that "a significant fraction" of the water on Earth was inherited from interstellar space, and was there before the Sun was formed some 4.6 billion years ago.
Researchers can tell where the water comes from by examining the ratio of hydrogen to deuterium, a heavy isotope of hydrogen, in water molecules.
Water or ice that comes from interstellar space has a high ratio of deuterium to hydrogen, because it forms at such low temperatures.
But scientists have not known how much deuterium was removed in the process of the Sun's birth, or how much deuterium-rich water-ice the solar system would have produced when it was first born.
Scientists simulated the origin of a planet under conditions where all the deuterium from space ice has already been eliminated.
They found they could not reach the ratios of deuterium to hydrogen that are found in meteorite samples or Earth's ocean water.
Their findings suggests that at least some of the water in the solar system comes from outer space, and that water - an essential element for life on Earth - is not unique to our solar system.
"This is an important step forward in our quest to find out if life exists on other planets," said co-author Tim Harries, from the University of Exeter's Physics and Astronomy department.
"It raises the possibility that some exoplanets could house the right conditions, and water resources, for life to evolve."
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