Water - and life - on Earth probably did not start with comets, a study has said.
The first data from the European Space Agency's Rosetta probe indicates that icy rocks from the outer reaches of the solar system did not bring water to our planet.
It had been suggested previously that water may have arrived courtesy of these giant 'snow balls'.
But the probe, a portion of which landed on comet 67P/Churyumov-Gerasimenko in November and is still in orbit around the body, says that the water found on the comet is not like water on Earth.
Above: comet 67P where the Rosetta spacecraft's probe Philae landed in November
The study, published in the journal Science, suggests asteroids are a better bet for the origin of water. That's because the water on the comet has three times too much 'heavy' water with atoms of deuterium along with hydrogen and oxygen.
That suggests the 'signature' of the water is fundamentally different than that on Earth.
Prof Kathrin Altwegg, from the University of Bern in Switzerland and lead investigator on the Rosina instrument aboard Rosetta, said: "This ratio between heavy and light water is very characteristic. You cannot easily change it and it stays for a long time.
"If we compare the water in comets with the water we have on Earth, we can definitely say if the water on Earth is compatible with the water on comets."
The study adds to previous research on other comets from the Oort Cloud at the edge of our solar system indicating similar results. Comet 67P comes from a different region - the Kuiper Belt, outside Neptune - where the only other comet analysed (Hartley 2) appeared in studies to have similar water to Earth. That was partly why scientists were so excited to land on 67P.
But Pro Altwegg says that the findings indicate the comets there are, at best, a mixed bag -- and that asteroids formed closer to the sun are more likely to have the right type of water.
"We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water."
However, other scientists point out that there are still only two data points from the Kuiper Belt - and it is still possible that comets seeded water (and ultimately life) on our planet, and maybe even Mars too.