Most of the water that fills the world's oceans—and makes up more than half of the human body—may have started out as comets, not asteroids as current theories hold, according to new research. Scientists using the Herschel telescope found that unlike other comets studied, the comet Hartley 2 contains water with the same chemical signature as water on our planet, suggesting that cometary impacts brought the water that covered the rocky and dry early Earth, the BBC reports.
Hartley 2 is believed to have formed in the Kuiper belt, not far outside the solar system. Comets studied earlier came from the much more distant Oort cloud and had a different chemical composition, leading scientists to believe only 10% of Earth's water could have come from comets. "It was a big surprise when we saw the ratio was almost the same as what we find in the Earth's oceans," the lead researcher tells Reuters. "It means it is not true any more that a maximum of 10% of water could have come from comets. Now, in principle, all the water could have come from comets."