After a decades-long search, astronomers have finally found the very first molecule to ever form in the universe. The helium hydride ion (HeH+) has long been a key theoretical part of how the chemistry of the cosmos kicked off, but its apparent absence in space had scientists worried they'd gotten it wrong. Now the first unambiguous evidence of the molecule in a planetary nebula has been found.

The story goes that soon after the Big Bang, hydrogen and helium dominated the young universe. Eventually, neutral helium atoms combined with ionized hydrogen to form HeH+, which would have been the first type of molecule possible from the ingredients available. That in turn opened up different combinations that gradually snowballed into the extensive chemical library we know today.

That's all well and good in theory, but confirmation that that was how it happened has eluded scientists for decades. HeH+ was first produced in the lab way back in 1925, but despite the expectation that it should be naturally abundant in the interstellar medium, the molecule had never been observed until now.

"The chemistry of the universe began with HeH+," says Rolf Güsten, first author of the study. "The lack of definitive evidence of its very existence in interstellar space has been a dilemma for astronomy for a long time."

The problem is that HeH+ emits its strongest spectral line at a frequency of 2.01 THz, which Earth's atmosphere can block. To get past that, scientists have now used NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), which can scour the cosmos from its perch onboard a Boeing 747SP. Since this mobile observatory flies at an altitude of up to 45,000 ft (13,700 m), it can get an unobscured view of the required infrared wavelengths.

And sure enough, using SOFIA the team spotted the telltale spectral signature of HeH+ in the envelope of the planetary nebula NGC 7027.

"The discovery of HeH+ is a dramatic and beautiful demonstration of nature's tendency to form molecules", says David Neufeld, co-author of the study. "Despite the unpromising ingredients that are available, a mixture of hydrogen with the unreactive noble gas helium, and a harsh environment at thousands of degrees Celsius, a fragile molecule forms. Remarkably, this phenomenon can not only be observed by astronomers but also understood using theoretical models that we have developed."

The research was published in the journal Nature.

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