Around 380,000 years after the Big Bang, the plasma that was our universe cooled enough for nuclei and electrons to begin combining. Helium was the first atom, and helium atoms soon bonded with protons to make the universe’s first molecule, helium hydride (HeH+).
While chemists made HeH+ in the laboratory as early as 1925, it wasn’t until the 1970s that scientists suggested it might be found in the interstellar medium of space. Four decades later, astronomers report they have finally observed it (Nature 2019, DOI: 10.1038/s41586-019-1090-x). What they’re seeing may change models of chemical reactions in space.
The failure to spot HeH+ had been “nagging” scientists, University of Georgia physicist Phillip Stancil says. He was one of the authors of a 2002 paper that established HeH+ as the universe’s first molecule. Stancil says it bothered scientists that they couldn’t back up the 1970s models of the universe’s chemistry with experimental data. But now, he says, they can.
Rolf Güsten of the Max Planck Institute for Radio Astronomy and colleagues identified a spectral line characteristic of HeH+ in observations of the planetary nebula NGC 7027, one of the places HeH+ was thought likely to be found. The nebula’s HeH+ is not left over from the primordial universe. Planetary nebulae form after stars similar in size to our sun collapse, ejecting a shell of gas and leaving a white dwarf star at its center. The HeH+ Güsten’s group observed formed on the inside of that shell.
A few factors complicated previous efforts to observe HeH+, the first being its relative scarcity. The researchers identified the molecule’s ground state rotational transition at 149.1 µm. A carbon-hydrogen bond transition at 149.09 µm often masks this signal. It’s also obscured by water and other molecules in Earth’s atmosphere, making ground observations unlikely.
No flying or planned space telescopes are designed to observe this part of the spectrum. So in 2016, the researchers flew a spectrometer on a converted airplane operated by NASA that flies a telescope above the thickest part of the atmosphere, at an altitude higher than 12,000 meters. Güsten’s team used a heterodyne spectrometer, which compares incoming light to a reference light, a technique that can increase sensitivity. The researchers were able to distinguish the HeH+ transition from C–H lines in their data.
“It has been a long search,” says Jérôme Loreau, a physicist who has developed models of how HeH+ and other molecules developed in the early universe. One reason this observation is interesting, he says, “is that there is a discrepancy between the predicted abundance of HeH+ and the observed abundance.” Based on their observations, Güsten’s team calculated an HeH+ abundance that’s a factor of 3 higher than predicted by models of chemical reactions in space.
That mismatch indicates room for improvement in astrophysical models, Loreau says. Güsten agrees. “I would expect that models of the early universe will change,” he says.
Stancil is less certain. He says he has already made plans with a colleague to recalculate those HeH+ reaction rates in light of this finding. But he says a lot of assumptions went into the researchers’ calculations that might be masking something scientists don’t yet fully understand. Nevertheless, he says this discovery is likely to spur new modeling and lab work that could improve our understanding of chemistry in space and the early universe.