Results from the ESA's Rosetta mission, published in the journal Science, suggest that Earth's water didn't come from comets, as was previously thought.
The Rosetta data suggests that most of Earth's water may have come from asteroids, since asteroid characteristics are similar to those of Earth's water, and asteroids are close to Earth. The researchers also found that the water on comet 67P/Churyumov-Gerasimenko, where the Rosetta's Philae lander touched down in November, isn't like the water on Earth — it has similar physical properties to H2O, but it's heavier in mass.
Scientists haven't ruled out the possibility that Earth's water is from comets, though. Comet 67P and another comet, Hartley 2, are the only comets whose water has been studied so far. And the water on Hartley 2 "appears to be exactly the same" as Earth's water, the BBC reports.