In a wild galaxy over half a billion light-years away, astronomers have detected molecular oxygen. It’s only the third such detection ever outside the Solar System — and the first outside the Milky Way.

Oxygen is the third most abundant element in the Universe, behind hydrogen (naturally) and helium. So its chemistry and abundance in interstellar clouds are important for understanding the role of molecular gas in galaxies.

Astronomers have searched for oxygen again and again, using millimetre astronomy, which detects the radio wavelengths emitted by molecules; and spectroscopy, which analyses the spectrum to look for wavelengths absorbed or emitted by specific molecules.