The Rosetta spacecraft has detected water coming off comet 67P, the rubber duck-shaped lump of ice and dust that it placed the Philae robotic lander on last month.

The presence of water is not a surprise, but what has wrong-footed researchers is the makeup of the water, which is nothing like that seen on Earth.

Measurements from Rosetta’s Rosina instrument found that water on comet 67P /Churyumov-Gerasimenko contains about three times more deuterium – a heavy form of hydrogen – than water on Earth.

The discovery seems to overturn the theory that Earth got its water, and so its ability to harbour life, from water-bearing comets that slammed into the planet during its early history.

Comet 67P is thought to have come from what is called the Kuiper belt, a broad band of frozen bodies that begins beyond the orbit of Neptune. The main asteroid belt contains more rocky objects that circle the sun between the orbits of Mars and Jupiter.

Kathrin Altwegg at the University of Bern said that rather than comets ferrying water to Earth, it may have arrived onboard asteroids instead. Details of the discovery are reported in the journal, Science.

“Today asteroids have very limited water, that’s clear. But that was probably not always the case, said Altwegg. In the earliest period of the solar system, 3.8bn years ago, asteroids are thought to have crashed into Earth regularly in what is called the late heavy bombardment. “At that time, asteroids could well have had much more water than they have today,” Altwegg said.

The Rosina instrument measured water coming off the comet as it flew around the body. Scientists plan to take more measurements as the comet nears the sun and its begins to spew more water vapour and dust out into space.

Measurements from other comets have found water with similar deuterium contents to that on Earth. But the strange composition of comet 67P’s water suggests that the picture of comets bringing water to Earth is too simplistic. “In the end, Earth’s oceans are probably a mix of many things,” Altwegg told New Scientist magazine.

Matt Taylor, Rosetta project scientist, said that teams at the European Space Agency were still looking for the Philae lander, which ran out of battery power soon after it bounced down onto the comet’s surface.

“Once we get the identification of where the lander is, it will give us a better fix on what we believe the illumination conditions are and a better idea of when we would expect the lander to have sufficient illumination to be able to start charging its batteries and come back online,” he said.

If the Philae lander can summon enough power from the feeble sunlight to come out of hibernation, a British instrument called Ptolemy could help to verify whether the water measurements from Rosetta are accurate. But another option is for Rosetta to fly through the jets of debris that will grow as the comet closes in on the sun.

“Part of our future plan is to do this,” said Taylor. “We’re focusing on what is known as an activity campaign to try and fly directly through a jet and we aim to do that hopefully around summertime next year.”