A distant cloud of gas and dust in space has been found to contain a significant amount of pyrene, which is classified as a large carbon-rich molecule known as a polycyclic aromatic hydrocarbon (PAH). This discovery hints that pyrene could be a key contributor to the carbon present in our solar system.
A research team from MIT has uncovered that a remote interstellar cloud is rich in pyrene, a large molecule consisting of carbon known as a polycyclic aromatic hydrocarbon (PAH).
The detection of pyrene in this distant cloud, which resembles the dust and gas that formed our solar system, implies that pyrene might have played a crucial role in supplying carbon to our solar system. This idea is bolstered by recent findings showing that pyrene is prevalent in samples from the near-Earth asteroid Ryugu.
Brett McGuire, an MIT chemistry assistant professor, notes, “One of the major questions regarding star and planet formation is: To what extent is the chemical composition of the original molecular cloud inherited in forming the solar system? Our observations reveal a connection between the beginning and end stages, providing compelling evidence that materials from the ancient molecular cloud contribute to the ice, dust, and rocky bodies within our solar system.”
Because of its symmetrical structure, pyrene is not detectable by existing radio astronomy methods that have identified about 95 percent of molecules in space. Instead, the researchers had to identify an isomer of cyanopyrene—a modified version of pyrene that loses its symmetry due to a reaction with cyanide. This molecule was observed in the far-off cloud known as TMC-1 with the help of the 100-meter Green Bank Telescope (GBT) located in West Virginia.
McGuire and Ilsa Cooke, an assistant chemistry professor at the University of British Columbia, are the senior authors of a study that details these findings, which will be published in Science. Gabi Wenzel, a postdoctoral researcher at MIT, is the lead author of the paper.
Carbon in space
Polycyclic aromatic hydrocarbons (PAHs), which are compounds made up of rings of carbon atoms, are thought to account for 10 to 25 percent of the carbon found in space. Over the past 40 years, researchers have utilized infrared telescopes to spot characteristics indicative of PAHs in space, but these techniques haven’t been able to pinpoint specific PAH types definitively.
Wenzel explains, “Since the PAH hypothesis emerged in the 1980s, many have accepted the presence of PAHs in space. They’ve been discovered in meteorites, comets, and asteroid samples, but infrared spectroscopy has not allowed for conclusive identification of individual PAHs in the cosmos.”
In 2018, a team led by McGuire discovered benzonitrile—a six-carbon ring linked to a nitrile (carbon-nitrogen) group—in TMC-1. They used the GBT to identify molecules in space based on their rotational spectra, the unique light patterns emitted as molecules move through space. In 2021, his team became the first to detect individual PAHs in space, identifying two isomers of cyanonaphthalene, which consists of two fused rings with a nitrile group attached to one.
On Earth, PAHs are often produced as byproducts of fossil fuel combustion and can also be found in char marks from grilled food. Their presence in TMC-1, which has a temperature of only around 10 kelvins, indicates that they can form at very low temperatures.
The finding of PAHs in meteorites, asteroids, and comets has led many in the scientific community to suggest they are a primary source of carbon that contributed to the formation of our solar system. In 2023, Japanese researchers discovered substantial amounts of pyrene in samples from the asteroid Ryugu taken during the Hayabusa2 mission, along with smaller PAHs like naphthalene.
This discovery inspired McGuire and his colleagues to search for pyrene in TMC-1. Pyrene has four rings and is larger than any other PAHs previously identified in space; in fact, it’s the third-largest molecule found in the cosmos and the largest detected through radio astronomy.
Before searching for these molecules in space, the team synthesized cyanopyrene in a lab setting. The cyano or nitrile group is key for creating a detectable signal for radio telescopes. This synthesis was conducted by MIT postdoc Shuo Zhang under the guidance of Alison Wendlandt, an associate professor of chemistry at MIT.
After synthesizing, the researchers analyzed the emitted signals from these molecules in the lab, which matched the signals detected in space.
Using the GBT, the researchers discovered these signatures throughout TMC-1, finding that cyanopyrene constitutes about 0.1 percent of the total carbon in the cloud. While this may seem minimal, it becomes significant when considering the thousands of carbon-containing molecules present in space, according to McGuire.
McGuire states, “Although 0.1 percent seems small, most carbon is found in carbon monoxide (CO)—the second most common molecule in the universe after molecular hydrogen. Excluding CO, one in every few hundred carbon atoms is pyrene. When accounting for the vast number of different carbon molecules out there, where most contain many carbon atoms, about one in a few hundred being pyrene represents an extraordinarily high abundance—essentially an impressive reservoir of carbon. It’s like an interstellar safe haven for stability.”
Ewine van Dishoeck, a professor of molecular astrophysics at Leiden Observatory in the Netherlands, described the discovery as “unexpected and exciting.”
She elaborates, “It builds upon their earlier findings of smaller aromatic molecules, but moving to the pyrene family is a significant leap. This not only underscores that a considerable part of carbon exists within these molecules but also suggests different pathways for the formation of aromatics than previously considered,” says van Dishoeck, who was not part of the study.
An abundance of pyrene
Interstellar clouds like TMC-1 are potential sites for star formation, as accumulations of dust and gas come together and generate heat. Planets, asteroids, and comets form from the gas and dust surrounding nascent stars. Although we cannot directly observe the original interstellar cloud that birthed our solar system, the identification of pyrene in TMC-1 and its abundance in the asteroid Ryugu suggests that pyrene might have significantly contributed to the carbon in our solar system.
McGuire asserts, “We now possess, I would argue, the strongest evidence ever corroborating direct molecular inheritance from the cold cloud right through to the actual rocks in our solar system.”
The team plans to search for even larger PAH molecules in TMC-1 and aims to explore whether the pyrene found there originated within the cold cloud or if it came from outside sources, perhaps from the energetic reaction processes associated with dying stars.
This research received support from several sources, including a Beckman Foundation Young Investigator Award, the Schmidt Family Futures Foundation, the U.S. National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, the Goddard Center for Astrobiology, and the NASA Planetary Science Division Internal Scientist Funding Program.
