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.