Molecular oxygen has been found on a comet, the first time such a discovery has ever been made, researchers are reporting.

The oxygen, found by the European Space Agency's Rosetta spacecraft in gases surrounding Comet 67P/Churyumov-Gerasimenko, has probably been there since the comet formed and may predate the creation of our solar system, they say.

"We believe this oxygen is primordial, which means it is older than our Solar System," says Andre Beiler of the University of Michigan.

The finding may yield new insights into the chemistry that occurred as our solar system formed around 4.6 billions years ago, the researchers suggest in their study published in the journal Nature.

The finding came as a surprise, as scientists had assumed oxygen would not be present in comets like 67P because it mixes so easily with other elements.

"We never thought that oxygen could 'survive' for billions of years" in a pure state, says study co-author Kathrin Altwegg of the University of Bern in Switzerland.

"It is the most surprising discovery we have made so far, because oxygen was not among the molecules expected in a cometary coma," she says.

An instrument on the Rosetta spacecraft known as the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, or Rosina, made the discovery.

Scientists say that in the coma — the cloud of gas surrounding a comet — they expected to find water, carbon dioxide and carbon monoxide.

While all three of those compounds contain oxygen atoms, free molecular oxygen — like that found in the Earth's atmosphere — was a surprising discovery, they say.

The finding will likely challenge some current mainstream theories about how the solar system formed and the elements involved, says Altwegg.

"This evidence of oxygen as an ancient substance will likely discredit some theoretical models of the formation of our Solar System," she says.

The oxygen in Comet 67P was probably created by high-energy particles colliding with bits of ice and creating hydrogen and oxygen in the cold, dense region that would one day host the birth of our sun, the researchers say.

Molecular oxygen that has already been detected of several moons of Jupiter and Saturn likely is being created in the same process of high-energy particles colliding with oxygen-containing compounds and freeing it.

Comets are prized as study candidates because their chemistry carries relatively pure evidence of their formation, providing hints about the early history of our solar system.