The water molecules being released by comet 67P/Churyumov-Gerasimenko aren’t a perfect match for the ones on Earth, according to new data published Wednesday in the journal Science.

Scientists are always trying to figure out where the water on Earth came from. After all, water seems to be a key ingredient in life as we know it. And when Earth was first formed, it was probably too hot for any liquid water to survive. The water present today almost certainly came from somewhere out in space.

Several leading theories hold that comets brought water molecules to Earth when they collided with the planet, but the Rosetta orbiter’s readings seem to contradict that.

After using Rosetta’s ROSINA instrument to sniff out the molecular composition of the comet’s vapor emissions, mission scientist Kathrin Altwegg and her team compared the alien water to what we might find on Earth. Analysis of other comets has been mixed: Some seem to have water identical to our own, and others don’t. But comet 67P/Churyumov-Gerasimenko’s water has a much higher ratio of deuterium — a heavier version of hydrogen — than Earth water. The researchers have determined that a comet like this one definitely couldn’t have seeded Earth with the water it has now.

But this isn’t the end of the story. It’s still possible that the comet has more Earthlike water tucked away somewhere else. If that’s the case, scientists might see it once the comet gets closer to the sun and more ice evaporates.

Even if comet 67P/Churyumov-Gerasimenko doesn’t have a lick of Earthlike water, this data won’t be enough to summarily dismiss the idea that any comet could have brought those molecules to Earth.

It’s more likely, however, that asteroids were the water bearers. The only comets found to have Earthlike water have been very far away, and research suggests that nearby asteroids fit the bill.

Asteroids have less water piece-for-piece than comets do, but a bombardment of many waterlogged asteroids could have taken the place of one massive comet collision.

“We knew that Rosetta’s in situ analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science, and this outstanding observation certainly adds fuel to the debate about the origin of Earth’s water,” said Matt Taylor, ESA’s Rosetta project scientist.

Philae, Rosetta’s lander, shut down just a few days after touching down on the comet in November. But Rosetta continues to orbit and collect data. During the next 12 months, the orbiter is going to follow its comet closer and closer to the sun, observing what changes the star may inflict upon the dusty ball of ice.