The Rosetta spacecraft has found molecular oxygen on a comet, a "surprising" discovery scientists say could change our understanding of how the solar system formed.

In a paper published in the journal Nature on Wednesday, scientists examined the composition of comet 67P/Cheryumov-Gerasimenko's "neutral gas coma," which is the cloud of gas surrounding the comet's nucleus.

According to the paper, most comet comas are comprised of up to 95 per cent molecular water (H20) and molecular carbon dioxide (CO2), with the remaining five per cent comprised of other molecules.

Until now, however, molecular oxygen hadn't been detected in a cometary coma.

"It is the most surprising discovery we have made so far in 67P," University of Bern professor Kathrin Altwegg, one of the paper's authors, said at a press conference. "Oxygen was not among the molecules suspected in a cometary coma."

Using measurements from the comet from between September, 2014, and March, 2015, the report's authors found what they call "an abundance of O2 (molecular oxygen)" in 67P's coma.

The scientists observed the ratio of molecular oxygen to hydrogen for several months, and found that it was constant.

This suggested that the molecular oxygen was present "throughout the whole body" of the comet, not just in an outer layer that would burn off over time, study co-author Andre Bieler said.

"The fact that (oxygen was) incorporated throughout the whole body led us to the idea that it must have been primordial," he said. "(The oxygen) must have been present before the formation or at the formation of the comet."

According to the report, scientists found this surprising because "current solar system formation models do not predict conditions that would allow this to occur."

Oxygen is a highly volatile molecule, Altwegg said, so it was previously believed that oxygen would have reacted with hydrogen to form H2O during or after the comet's birth, instead of remaining on 67P.

Because of this, scientists were surprised to find the oxygen could have been incorporated into the comet's nucleus when 67P formed, and that it remains there today.

"The first time we really saw it, I think we all went a little bit into denial because it was really not expected," Altwegg said.

The scientists went over the data again to be certain what they observed was real.

"We did everything to make sure that it's not an instrumental effect, not coming from the … spacecraft," Altwegg said. "(We found) that this is real, it's really cometary."

Altwegg said the discovery isn't likely to directly affect our understanding of life on Earth, but it could impact how we identify planets capable of carrying life.

"So far the combination of methane and O2 was a hint that you have life underneath it," Altwegg said. "On the comet, we have both. We have methane and O2, but we don't have life. So it's probably not a very good bio signature to have O2."