Scientists are pretty sure that Earth's water didn't originate on Earth itself, so where did it come from, exactly? Many believe the source is water-rich comets that bashed into our planet billions of years ago. However, new observations from the Rosetta spacecraft have weakened that theory. After it scanned the water vapor streaming from Comet 67P (above), ESA scientists found that there was three times more deuterium (heavy water) than found on Earth. That's significant, because of 11 comets measured to date, only one has the same water we do -- Comet 103P, a Jupiter-class (Kuiper Belt) comet analyzed by the ESA's Herschel telescope in 2011.
That led scientists to believe that all Jupiter-class comets (found in an orbit closer to the sun than Oort Cloud comets, as shown below) contained Earth-like water, but Rosetta's findings have likely ruled out that idea. Rosetta's Comet 67P, a Jupiter-class comet itself, contains the highest ratio of heavy water for any comet ever observed in either comet belt. That leaves asteroids as the most likely donor of water on Earth. Though they contain a much lower percentage of water than comets, the composition of that water is the same as Earth's. However, it would've taken a much larger number of collisions to have produced our oceans.
That said, Rosetta's measurements haven't provided definitive proof of where our water comes from. One researcher told the BBC that future measurements of both asteroids and comets between Mars and Jupiter are needed. Another pointed out that Rosetta's observation don't give a true picture of Comet 67P's water composition, because the amount of heavy water varies as gas escapes its surface. What's really needed are measurements from the Philae Lander's instruments -- and unfortunately, Philae is currently stuck in limbo.