Air surrounding the comet where a European probe landed last year is rich with oxygen, scientists have learned.
The surprise discovery may force a rethink of theories about the origins of the Solar System - but does not imply the presence of life.
Experts controlling the European Space Agency's Rosetta orbiter discovered that free oxygen is the fourth most common gas in the atmosphere around Comet 67P Churyumov-Gerasimenko.
Its other constituents are water vapour, carbon monoxide and carbon dioxide.
Oxygen is highly reactive and according to current theories should not exist on its own in such quantities.
Over vast amounts of time, most of Comet 67P's oxygen should by now have combined with hydrogen to form water, it was thought.
Professor Kathrin Altwegg, project leader for Rosetta's Rosina mass spectrometer instrument, said: "We had never thought that oxygen could 'survive' for billions of years without combining with other substances."
While microbes and plants are responsible for most of Earth's oxygen, the new discovery does not mean that Comet 67P is teeming with life.
Instead, scientists believe the comet's oxygen originated very early, before the solar system had even finished forming.
High energy particles are thought to have freed the oxygen by striking grains of ice in the cold and dense birthplace of the solar system, known as a "dark nebula".
The oxygen was incorporated into the comet nucleus when it was created some 4.6 billion years ago and has remained ever since, according to the researchers writing in the journal Nature.
Prof Altwegg added: "This evidence of oxygen as an ancient substance will likely discredit some theoretical models of the formation of the Solar System."
In November last year, the Rosetta mission made history by deploying a small probe to land on the comet.
The Philae lander bounced onto 67P's surface before coming to rest at the foot of a cliff or crater wall.
Although the craft has maintained intermittent contact with Earth, it has collected a wealth of data for scientists.
The probe travelled with the comet as it hurtled to its closest point to the sun on 13 August.