Space is famously empty. The cold vacuum of space—or more specifically, the interstellar medium—lacks much of anything, including the air needed to conduct sound. But it isn’t quite completely empty. While it’s vacant compared to what we experience in daily life, there are occasional atoms and molecules spread throughout it.
Those atoms and molecules mean that there is chemistry in space, although it doesn’t always resemble the dense, warm reactions that routinely occur in a chemist’s test tubes. One aspect of chemistry in space that researchers are interested in is the formation of polycyclic aromatic hydrocarbons (PAHs), which are molecules of carbon and hydrogen that make a broad array of chemicals on earth and in the void of space. Researchers have seen signs of light interacting with a variety of these molecules in space and being absorbed—leaving a distinctive fingerprint in the remaining light that reaches Earth. These molecules are estimated to contain somewhere between a tenth and a quarter of the carbon spread across the interstellar medium, and the molecules’ foundational building blocks are benzene (C6H6)—a ring of six carbon atoms, each holding a hydrogen atom.
Since 1999, researchers have had a model that they thought explained how benzene formed from smaller molecules. However, the challenges of performing experiments at the low temperatures and densities involved in mimicking the conditions in the interstellar medium have meant that researchers have relied on their theoretical understanding of the process and haven’t thoroughly tested it in experiments.
Now, JILA Fellow and University of Colorado Boulder Physics Professor Heather Lewandowski and members of her lab have used tools developed in physics laboratories to recreate the necessary conditions and have investigated how the chemistry plays out. The team described their experiment in an article published in the journal Nature Astronomy in May 2025. When they tested the process, the first steps played out as expected, but then they were surprised to find that the benzene failed to form at the final step. Their results give scientists a new window into how chemistry occurs in the interstellar medium and reopens the question of how carbon gets caught up in PAHs throughout space.
The key to recreating the chemistry occurring in the interstellar medium was creating a vacuum in a chamber and using lasers to cool molecules and hold them in place in the vacated space. This required the researchers to look at just a small number of molecules and to set aside the beakers and test tubes that are stereotypical of chemistry and instead rely on large metal chambers, air pumps, laser beams and many mirrors and lenses.
“It's a laboratory full of lasers, and vacuum chambers, and optics,” Lewandowski says. “It fills up half a room to be able to cool down these hundred little molecules.”
Selecting the right color of laser and aligning the beams correctly allows the researchers to suspend—trap—particles in a vacuum chamber as well as cool them down through a process called laser cooling. Laser cooling relies on the fact that light can give atoms and molecules a shove to slow them down and that the interaction can be tailored to depend on how the particles are moving. Carefully applied, laser cooling can get molecules down to temperatures just above absolute zero.
“Laser cooling and trapping has really been in the domain of physicists,” Lewandowski says. “The nice thing about JILA is we have physicists and chemists working together. In my own group, we have both backgrounds, and so we have the tools now that can answer these questions that really chemists didn't have the technology to tackle and physicists didn't know it was an interesting question to answer.”
These techniques allow them to focus on a small number of molecules and get a close look at the interactions that normally are obscured in a chaos of many reactions occurring rapidly and simultaneously.
With the equipment creating the needed conditions, the group started following the proposed recipe for creating benzene in the interstellar medium. The recipe’s main ingredient is a molecule of two carbon atoms and two hydrogen atoms, called acetylene (C2H2). The first step is mixing acetylene with molecules containing two nitrogen atoms and one hydrogen atom (N2H+). The nitrogen atoms can provide their hydrogen atom to create new molecules with two carbon and three hydrogen atoms. That opens the door to two more steps of interactions with acetylene molecules to produce a molecule with six carbon atoms and five hydrogen atoms (C6H5+)—just one hydrogen short of the target benzene ring. The exact behavior of this molecule is not thoroughly understood, but the established recipe proposed that it could form benzene by capturing a molecule made from a pair of hydrogens and then letting the excess atoms go.
The team supplied just enough of the needed ingredients in the chamber so that it was improbable that more than two molecules would be reacting at a time. Using laser cooling, they cooled the molecules in the chamber down to just a few degrees Kelvin. This setup let them recreate what happens when two lonely molecules finally come together in space and get the chance to interact.
The group repeatedly ran the experiment, stopping after different amounts of time to eject the cloud of molecules and check which molecules had been formed. They saw the mixture progress through the expected steps of the recipe. They observed increases of various molecules as they were created and then decreases as they were consumed in the construction of even larger molecules. But as they waited progressively longer and longer, they never caught sight of any benzene rings. The mixture in the chamber eventually just reached a steady amount of C6H5+, and the final step of the recipe failed to occur.
“Initially we were very confused—and a little irritated—because we could never get the final reaction to happen,” says JILA postdoctoral researcher G. Stephen Kocheril, the lead author of the paper.
After performing several runs of the experiment and analyzing the data, the team concluded that the expected chain of events wasn’t happening and there must be something else occurring to produce all the benzene in space.
“None of the models now actually predict what's out there,” Lewandowski says. “If you look at observations of how many of these molecules we have out there, no model works. So we sort of said, ‘this model isn't it.’ We don't have a new model yet; that's what we're working on now. So it was kind of big for the community because it changed how larger and larger carbon-containing molecules are formed in space.”
Moving beyond the old explanation gives chemists insights into how they should think about the formation of these molecules and provides astronomers with new clues about which molecules they should be keeping an eye out for if they want to understand the chemistry happening out in the interstellar medium.
Written by Bailey Bedford, Freelance Science Communicator