When the Universe was still very young, only a few kinds of atoms existed. Astrophysicists believe that around 100,000 years after the Big Bang, ionized hydrogen and neutral helium atoms combined to make the helium hydride ion (HeH+) for the first time. This molecule should be present in some parts of the modern Universe, but it has never been detected in space — until now. A team of researchers from the United States, Germany and France discovered its signature in NGC 7027, a planetary nebula located approximately 3,000 light-years away in the constellation Cygnus, using NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA).
Güsten et al detected the first type of molecule that ever formed in the Universe — HeH+. This discovery confirms a key part of our basic understanding of the early Universe and how it evolved over billions of years into the complex chemistry of today. Image credit: NASA’s Ames Research Center.
“HeH+ was lurking out there, but we needed the right instruments making observations in the right position — and SOFIA was able to do that perfectly,” said Dr. Harold Yorke, director of the SOFIA Science Center.
Today, the Universe is filled with large, complex structures such as planets, stars and galaxies. But more than 13 billion years ago, following the Big Bang, the early Universe was hot, and all that existed were a few types of atoms, mostly helium and hydrogen.
As atoms combined to form the first molecules, the Universe was finally able to cool and began to take shape. Astrophysicists have inferred that HeH+ was this first, primordial molecule.
Once cooling began, hydrogen atoms could interact with HeH+, leading to the creation of molecular hydrogen — the molecule primarily responsible for the formation of the first stars.
Stars went on to forge all the elements that make up our rich, chemical cosmos of today.
The problem, though, is that scientists could not find HeH+ in space. This first step in the birth of chemistry was unproven, until now.
“The lack of evidence of the very existence of HeH+ in interstellar space was a dilemma for astronomy for decades,” said Dr. Rolf Guesten, a researcher at the Max Planck Institute for Radio Astronomy, Germany.
In 1925, chemists were able to create HeH+ in a laboratory by coaxing the helium to share one of its electrons with a hydrogen ion.
Then, in the late 1970s, astronomers studying NGC 7027 thought that this environment might be just right to form HeH+. UV radiation and heat from the aging star create conditions suitable for HeH+ to form. But their observations were inconclusive. Subsequent efforts hinted it could be there, but HeH+ continued to elude detection.
In 2016, scientists turned to SOFIA for help. Flying up to 45,000 feet (13.7 km), SOFIA makes observations above the interfering layers of Earth’s atmosphere. But it has a benefit space telescopes don’t — it returns after every flight.
“We’re able to change instruments and install the latest technology. This flexibility allows us to improve observations and respond to the most pressing questions that scientists want answered,” said SOFIA deputy project scientist Dr. Naseem Rangwala.
The study was published in the April 11 issue of the journal Nature.
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Rolf Güsten et al. 2019. Astrophysical detection of the helium hydride ion HeH+. Nature 568: 357-359; doi: 10.1038/s41586-019-1090-x
