The instruments of the European Space Agency's Rosetta probe has identified oxygen molecules in the atmosphere, or coma, of comet 67P/Churyumov-Gerasimenko. It's the first time molecular oxygen has been found on a comet.

According to the recent analysis, O2 is the fourth most abundant gas surrounding the comet, behind water vapor, carbon monoxide and carbon dioxide. Scientists didn't expect to find it, assuming oxygen would only exist as H2O or CO2. But there it is, forcing scientists to rethink the search for alien organisms and the evolution of the solar system.

"It's actually the most surprising discovery we have made so far on 67P because oxygen was not among the molecules expected in a cometary coma," Kathrin Altwegg, an Rosetta scientist and researcher with the University of Bern, told The Guardian.

Oxygen is abundant throughout the universe, but because it reacts easily with other gases and particles, it is elusive. The oxygen on Rosetta's comet isn't just passing through. Rosetta detected consistent O2 levels between September 2014 and March 2015, suggesting the molecules are present throughout the comet's body, not just in its outer layers. As it sheds bits and pieces of itself, and as its inner gases sublimate, oxygen is continually added to its atmosphere.

Scientists are now working to figure out where exactly the O2 comes from. The fact that oxygen is present throughout 67P's composition suggests the molecules were incorporated early in the comet's formation, some 4.6 billion years ago.

The findings, detailed in the journal Nature, discredit theories that suggest all matter in the solar system was heated and cooled early in the evolution of the planetary system. The theory that comets offer a snapshot of the early universe, however, is reinforced by the new data.

"This is still consistent with estimates for the comet formation conditions in the outer solar nebula," Ewine van Dishoeck, an astronomer at the Leiden Observatory in the Netherlands, said in a press release.

Because the oxygen was successfully preserved in the comet, researchers believe the comet's formation may have been a rather delicate affair -- not one of violent collisions.

"Regardless of how it was made, the O2 was also somehow protected during the accretion stage of the comet: this must have happened gently to avoid the O2 being destroyed by further chemical reactions," added Altwegg.

"This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of Solar System evolution," concluded Matt Taylor, lead scientists on the Rosetta mission.