To survive, humans breathe in oxygen, a gas vital to sustain life.
It was previously thought that oxygen can only be found on Earth.
Not anymore.
European scientists from the Rosetta mission have found oxygen—"a lot of oxygen"—in the atmosphere of comet 67P/Churyumov-Gerasimenko.
The Los Angeles Times said it was a discovery "that could change our understanding of how the solar system formed."
The molecular oxygen (O2) was detected by the ROSINA mass spectrometer, one of the instruments aboard the Rosetta spacecraft that has been traveling with the comet since August 2014.
The finding stunned the scientists. "The first time we saw it, we all went a little bit into denial because molecular oxygen was really not expected to be found on a comet," said Kathrin Altwegg of the University of Bern in Switzerland, the project leader for ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis.
"It does not sound that spectacular, but it is actually the most surprising discovery we have made so far on 67P," she said of the comet, which is about to pass the orbit of Mars.
Molecular oxygen is rarely seen outside our planet. Astronomers have detected molecular oxygen outside the solar system only twice, and never on a comet, the Los Angeles Times said.
Altwegg said the ROSINA instrument found that oxygen is the fourth most abundant compound in the gaseous coma of the comet after water, carbon monoxide and carbon dioxide. "It is not only that we have oxygen, we have a lot of oxygen," she said.
The researchers could not explain why the comet manages to retain its oxygen despite getting fried when it approaches the sun, where the intense radiation causes it to shed "between one and 10 meters from its circumference."
In a Nature paper describing the discovery, the researchers noted that the ratio of water to oxygen in the comet's atmosphere remained the same over the many months that ROSINA was collecting measurements.
The scientists said this could mean that the oxygen is present throughout the body of the comet and not just at its surface.
And if that is true, the next logical conclusion is that molecular oxygen was present at the time the comet formed, the researchers said.
"The implication that this is primordial material is very interesting. It starts to give you a peek into the environment in which the solar system formed," the Los Angeles Times quoted Christopher Snead, who studies comet dust at UCLA, as saying.
But despite finding oxygen in the comet's coma, instruments have not detected even primordial life on 67P. This leads the scientists to speculate that looking for molecular oxygen outside Earth may not be the best way to look for extraterrestrial life.
"As far as we knew the combination of methane and O2 [oxygen] was a hint that you had life, but on our comet we have both methane and O2, but we don't have life, so it is probably not a very good bio-signature," Altwegg said.
