ESA’s Herschel space observatory has found molecules of oxygen in a nearby star-forming cloud. This is the first undisputed detection of oxygen molecules in space. It concludes a long search but also leaves questions unanswered.

The oxygen molecules have been found in the nearby Orion star-forming complex. While atomic oxygen has been long known in warm regions of space, previous missions looking for the molecular variety – two atoms of oxygen bonded together – came up largely empty-handed.

Even the observed amount of atomic oxygen is far less than that expected and this created an oxygen ‘accounting problem’ that can be roughly voiced as “where is all the oxygen hiding in the cold clouds?”

NASA’s Submillimetre Wave Astronomy Satellite and Sweden’s Odin mission have both searched for molecular oxygen and established that its abundance is dramatically lower than expected.

One possibility put forward to explain this was that oxygen atoms freeze onto tiny dust grains found floating in space and are converted to water ice, effectively removing them from sight.

If this is true, the ice should evaporate in warmer regions of the cosmos, returning water to the gas and allowing molecular oxygen to form and to be seen.