A groundbreaking discovery by MIT researchers has unveiled a significant finding: a distant interstellar cloud rich in pyrene, a large polycyclic aromatic hydrocarbon (PAH) composed of carbon atoms. This exciting revelation sheds light on the origins of carbon within our solar system.
The presence of pyrene in this cloud, reminiscent of the primordial dust and gas that birthed our solar system, corroborates the theory that it could have supplied much of our solar system’s carbon. Supporting evidence from recent investigations of samples retrieved from the near-Earth asteroid Ryugu highlights significant pyrene deposits there as well.
“Understanding how much of the chemical makeup of early molecular clouds influences the solar system’s formation is a pivotal question in astrophysics,” explains Brett McGuire, Assistant Professor of Chemistry at MIT. He emphasizes, “Our findings reveal a remarkable correlation between these primitive clouds and the materials that constitute our solar system.”
Detecting pyrene was challenging due to its symmetrical structure, which eludes traditional radio astronomy methods that have successfully identified around 95% of molecules in space. Instead, researchers identified cyanopyrene, a reactive isomer of pyrene altered by cyanide, within the TMC-1 cloud using the Green Bank Telescope (GBT) in West Virginia.
The groundbreaking study, which has been published in Science, features lead author Gabi Wenzel and senior authors McGuire and Ilsa Cooke, Assistant Professor of Chemistry at the University of British Columbia. You can read the full paper here.
The Role of Carbon in Space
Polycyclic aromatic hydrocarbons like pyrene are believed to store 10 to 25 percent of the universe’s carbon. While infrared telescopes have detected PAHs in the cosmos for over four decades, pinpointing exact molecular types remained elusive, according to Wenzel.
“While PAHs have been acknowledged in space since the 1980s, identifying specific types has presented challenges,” she notes.
Back in 2018, McGuire’s team made headlines when they identified benzonitrile in TMC-1. Utilizing the GBT’s ability to study molecules through their rotational spectra, they made another breakthrough in 2021 by observing the first individual PAHs in space: isomers of cyanonaphthalene.
On Earth, PAHs can be found as byproducts of fossil fuel combustion and even in charred food. Their detection in the frigid environment of TMC-1, which reaches only about 10 kelvins, hints that these molecules can form under extraordinarily low temperatures.
Furthermore, the discovery of PAHs in meteorites, comets, and asteroids has led scientists to hypothesize that they may be vital sources of carbon for our solar system’s formation. Recently, large amounts of pyrene were identified in samples from asteroid Ryugu, fueling further exploration by McGuire’s team in the TMC-1 cloud.
Strikingly, pyrene is the largest PAH recognized in space, boasting four fused carbon rings, and it stands as the largest molecule ever detected via radio astronomy.
To facilitate their research, scientists synthesized cyanopyrene in the lab, equipping it with a cyano group that allows for radio detection. MIT postdoc Shuo Zhang, working under Alison Wendlandt, played a crucial role in this synthesis.
Once synthesized, the emitted signals from cyanopyrene in the lab were matched to those detected in TMC-1. The researchers discovered that cyanopyrene comprises approximately 0.1 percent of the carbon within the cloud. Although this percentage may seem modest, it carries weight considering the myriad carbon-containing molecules present in space.
“A seemingly small fraction can lead to substantial implications, especially when you consider carbon monoxide, the universe’s second most prevalent molecule,” McGuire explains. “If we set CO aside, one in every few hundred carbon atoms could be pyrene—indicating an extraordinary abundance.”
Ewine van Dishoeck, a molecular astrophysics professor at Leiden Observatory, remarked on the discovery as “unexpected and thrilling.” She believes it not only confirms that a considerable amount of carbon resides in these molecules but also suggests the existence of lesser-known formation pathways for aromatics.
The Abundant Presence of Pyrene
Clouds like TMC-1 may ultimately coalesce into stars, leading to the birth of planets, asteroids, and comets from surrounding gas and dust. Although we cannot directly observe the cloud that gave rise to our solar system, the pyrene detected in TMC-1, coupled with findings from Ryugu, strongly suggests its role in the carbon genesis of our system.
“We now possess, without a doubt, compelling evidence of direct molecular inheritance spanning from the cold molecular cloud to the very rocks within our solar system,” McGuire asserts.
Future research endeavors include the search for even larger PAH molecules within TMC-1, as well as inquiries into whether the pyrene identified was formed within that cloud or originated from external cosmic events, possibly related to the explosive activity of dying stars.
This research was supported by multiple foundations, including the Beckman Foundation Young Investigator Award and the NASA Planetary Science Division Internal Scientist Funding Program.
Photo credit & article inspired by: Massachusetts Institute of Technology
