Molecules containing noble gases shouldn’t exist. By definition, these chemical elements — helium, neon, argon, krypton, xenon and radon — are the party poopers of the periodic table, huddling in the rightmost column and refusing to make molecules. Indeed, no one has ever seen any naturally occurring noble gas molecules on Earth. Earlier this decade, though, astronomers accidentally discovered one of these aloof elements in molecules in space.
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.
