Astronomers have discovered one of the largest carbon-based molecules found in deep space, located within the Taurus molecular cloud, 430 light-years from Earth. The finding is significant because it provides further clues that might help solve a longstanding mystery in astrochemistry: Where does carbon, the building block of life, come from?
The molecule, called pyrene, is made up of four fused planar rings of carbon. It's therefore categorized as a polycyclic aromatic hydrocarbon (PAH) — one of the most abundant complex molecules in the visible universe. PAHs were first detected in the 1960s, in meteorites known as carbonaceous chondrites, which are remnants from the primordial nebula that formed our solar system.
"One of the big questions in star and planet formation is how much of the chemical inventory from that early molecular cloud is inherited and forms the base components of the solar system?" Brett McGuire, an assistant professor of chemistry at the Massachusetts Institute of Technology, said in a statement.
PAHs are thought to make up roughly 20% of the carbon found in space and are present at different stages in the life of stars, from their formation to their death. Their stability and resilience to ultraviolet (UV) radiation makes them likely to survive even in the harsh conditions of deep space.
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The researchers say they began searching for pyrene and other PAHs in the Taurus cloud after pyrene was found in high levels in samples collected from the near-Earth asteroid Ryugu. Finding these molecules in the birthplace of our solar system provides a direct link astronomers have long been searching for.
"What we're looking at is the start and the end, and they're showing the same thing," said McGuire. "That's pretty strong evidence that this material from the early molecular cloud finds its way into the ice, dust and rocky bodies that make up our solar system."
The discovery was made using radio astronomy, a major subfield of astronomy that observes celestial objects, such as stars, planets, galaxies and clouds of dust, in the radio spectrum. By studying the radio waves originating from these sources, astronomers can learn about the compositions, structures and motions of particular targets.
Compared to other instruments used to identify molecules in space, radio telescopes offer the ability to observe individual molecules as opposed to general molecular groups. They do this by detecting the unique "fingerprints" of electromagnetic radiation a molecule emits or absorbs at specific frequencies where each molecule has a distinct set of rotational and vibrational energy levels. Characteristic radio waves are generated when the molecule transitions between these levels.
"This is now the seventh individual PAH identified in space since we first found one in 2021," said Ilsa Cooke, assistant professor in the UBC department of chemistry. "[PAHs] have similar chemical structures to the building blocks of life. By learning more about how these molecules form and are transported in space, we learn more about our own solar system and so, the life within it."
The astronomers estimated that pyrene accounted for about 0.1% of the carbon found in the cloud. "That is an absolutely massive abundance. An almost unbelievable sink of carbon. It’s an interstellar island of stability," said McGuire.
What was even more intriguing to the team, aside from finding pyrene in the origin place of our solar system, is the fact that the temperatures of the cloud were measured to be only 10 Kelvin (-263 degrees Celsius). On Earth, PAHs are formed during high temperature processes, namely through the combustion of fossil fuels. Finding them in this cold environment was therefore surprising. "Future work aims to explore whether PAHs can form somewhere that’s extremely cold, or whether they arrive from elsewhere in the universe, potentially via the death throes of an old star," said Cooke.
