Then, in 2019, observers reported finding a second kind of noble gas molecule, one they had sought for more than three decades and of a type that was the very first to form after the universe’s birth in the big bang. This newly found molecule lends insight into the chemistry of the early universe, before any stars began to shine or any galaxies had formed. The discovery may even help astronomers understand how the first stars arose.
Most chemical elements readily share electrons with other elements to make molecules, but noble gases normally don’t. “Noble gases are in some sense happy as they are,” says Peter Schilke, an astrophysicist at the University of Cologne in Germany. That’s because the outer shell of a noble gas atom already has its fill of electrons, so it won’t ordinarily exchange electrons to bond with other atoms and form molecules — at least, not here on Earth.
In retrospect, space seems the perfect place to seek noble gas molecules, because these gases abound in the cosmos. Helium is the second most common element in the universe, after hydrogen, and neon ranks fifth or sixth. And in interstellar space, where extreme temperatures and densities are the rule, noble gases do things they would never do on Earth. That includes forming molecules.
In addition to providing insight into the universe’s infancy, these exotic molecules tell scientists about the current conditions in the space between the stars — the gases that make up the interstellar medium — which is of intense interest to astronomers. “The interstellar medium is the place where stars and planetary systems are born,” says Maryvonne Gerin, an astrophysicist at the Observatory of Paris and coauthor of a 2016 Annual Review of Astronomy and Astrophysics article on interstellar molecules.
For decades astronomers have pursued one noble gas molecule in particular: helium hydride, or HeH+, made of the two most common elements in the universe and thus a good bet to exist in space. Though naturally occurring helium hydride has never been found on Earth, scientists were able to force the two atoms together in the lab almost a century ago.
So it seemed this combo would be the most likely quarry for astronomers as well. Instead, they were caught off guard by an even stranger molecule.
An interstellar embarrassment
Argon is more than 20 times as common in Earth’s atmosphere as carbon dioxide but gets far less press. In fact, it is the third most abundant gas in the air you breathe. Nitrogen and oxygen make up 78 percent and 21 percent of Earth’s atmosphere, respectively, while argon accounts for most of the remaining 1 percent.
But nobody was looking for an interstellar molecule containing argon. “It was basically a serendipitous discovery,” says University College London astrophysicist Mike Barlow, who led the team that accidentally found ArH+: argonium, which consists of argon and hydrogen.