A robot lab sent down to Comet 67P on November 12 last year, found no evidence that its nucleus was magnetised, they said.
The finding could sweep away a key theory on the formation of comets and other solar system bodies, said researcher Hans-Ulrich Auster.
It could mean that magnetic forces may not have played a role, as theorised by some, in a crucial stage of planet building.
The discovery was published in the journal Science and presented simultaneously at a meeting of the European Geosciences Union (EGU) in Vienna.
It was based on measurements sent home by the washing machine-sized lab, Philae, last November.
The probe's 20-kilometre (12-mile), seven-hour descent from orbiting mothership Rosetta ended with a rough landing — a mishap that turned out to be a boon for Auster's team.
The probe, which weighs 100 kilogrammes (220 pounds) on Earth but less than a feather in the comet's weak gravity, bounced off the hard surface several times before settling at an angle in a dark ditch.
"This complex trajectory turned out to be scientifically beneficial," said a European Space Agency (ESA) statement.
"The unplanned flight across the surface actually meant we could collect precise magnetic field measurements with Philae at the four points we made contact with, and at a range of heights above the surface," said Auster.
Philae had enough stored battery power for 60 hours of experiments and sent home reams of precious data before going into standby mode on November 15.
From analysis of the data, "we conclude that Comet 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object," said Auster.
Comets are clusters of primordial dust and ice orbiting the Sun in elliptical circuits.
The 1.3-billion-euro ($1.4-billion) Rosetta mission aims to unlock the secrets of comets, which astrophysicists believe may have "seeded" Earth with some of the ingredients for life.
Another keen area of interest is this: what comets can reveal about the role of magnetism in the formation of the solar system almost 4.6 billion years ago?
The idea is that the sun, asteroids, comets, moons and planets emerged from a swirling disc of gas and dust, much of it grains of magnetite, a form of iron.
At the micro scale, magnetic fields in the protoplanetary disc helped clump material together to create embryonic bodies, according to this hypothesis.
But how magnetism helped the accretion process thereafter is unclear.
Some theoreticians have suggested magnetism may have played a role in the intermediary body-building phase, before the object becomes large enough — hundreds of metres and then kilometres — for gravity to take over as the dominant force.
But Tuesday's results seem to disprove this.
"The theory that magnetic forces help to build planets becomes less likely," Auster, of the Technische Universitat Braunschweig in Germany, told AFP.
"If Comet 67P/Churyumov-Gerasimenko is representative of all cometary nuclei, then we suggest that magnetic forces are unlikely to have played a role in the accumulation of planetary building blocks greater than one metre (3.25 feet) in size," he added.
Karl-Heinz Glassmeier, a principal investigator on the Rosetta team, said magnetic fields appear to have been "much smaller in the early Solar System than previously thought, because if they would have been larger, we most probably would have seen a more strong magnetisation" on 67P.
Rosetta entered the comet's orbit last August after a six-billion-kilometre trek of more than a decade that required four flybys of Earth and Mars, using the planets' gravity as a slingshot to build up speed.
The comet is expected to reach its closest point to the sun, at a distance of 186 million kilometres on August 13.