The most ancient type of molecule in our universe has been detected in space, scientists have revealed, backing up theories of how the early chemistry of the universe developed after the big bang.

The positively charged molecule known as helium hydride is believed to have played a starring role in the early universe, forming when a helium atom shared its electrons with a hydrogen nucleus, or proton. Not only is it thought to be the first molecular bond, and first chemical compound, to have appeared as the universe cooled after the big bang, but it also opened up the path to the formation of molecules of hydrogen.

But while helium hydride was produced in the laboratory almost a century ago, it has proved elusive to detect in space, despite predictions that it should be present in gas clouds in which stars are born, as well as in gas expelled by stars as they die.

“Although [helium hydride] is of limited importance on Earth today, the chemistry of the universe began with this ion,” the authors write. “The lack of definitive evidence for its very existence in interstellar space has been a dilemma for astronomy.”

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Now experts say they have finally spotted helium hydride in a small but bright 600-year-old planetary nebula about 3,000 light years away in the constellation of Cygnus. While this helium hydride was formed by a different process to that in the early universe, the team say its presence backs up theories of what was going on at the “dawn of chemistry”, bringing a “decades-long search to a happy ending at last”.

Dr Jérôme Loreau, an expert in helium hydride from the Université Libre de Bruxelles, who was not involved in the study, said the findings were exciting, noting that although large organic molecules had been found in space, the much simpler helium hydride had proved difficult to track down.

“Another reason it is exciting is that HeH+ [helium hydride] is the first molecule ever to form in the universe, about 380,000 years after the big bang, in an era known as the recombination epoch,” said Loreau, adding that molecules that appeared in this period, as the universe cooled, later led to the formation of stars and galaxies. “It is therefore very exciting to have finally observed one of the building blocks of the molecular universe.”

One difficulty in detecting helium hydride is that telltale signs based on the light it emits after it is buffeted by electrons tend to be obscured by almost overlapping “signatures” from a different molecule composed of carbon and hydrogen, however, new instruments have turned up trumps.

Writing in the journal Nature, the team report they made their discovery by scrutinising a planetary nebula known as NGC 7027 in which layers of gas are being lost from its hot core – a white dwarf.

Data was collected using an instrument onboard a mission known as Sofia, a joint project between Nasa and the German Aerospace Center, in which a specially modified Boeing 747SP aircraft is flown very high up in the Earth’s atmosphere, up to about 13,700 metres (45,000ft), carrying a 2.7-metre telescope and other instruments. At this height, the instruments are above the vast majority of the Earth’s water vapour, which would otherwise obscure signals from other molecules.

Crucially, these instruments offer a very high resolution, allowing light from helium hydride to be distinguished from signatures of other molecules at very similar wavelengths.

Prof Phillip Stancil, an expert on helium hydride from the University of Georgia, also said the discovery was exciting, but pointed out only one “signature” of helium hydride was detected, and two are usually needed to be sure.

But Dr Rolf Güsten, from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and first author of the research, said the chance of the identification being wrong is negligible, adding that the findings resolve a longstanding conundrum.

“The lack of evidence of the very existence of helium hydride in the local universe has called into question our understanding of the chemistry in the early universe,” he said. “The detection reported here resolves such doubts.”