Polycyclic aromatic hydrocarbons (PAHs) are organic molecules of connected aromatic rings. While infrared emission bands indicate that PAHs are plentiful in space, only a few specific PAHs have been found in the interstellar medium.

An asteroid has led researchers to uncover the most significant molecule ever detected by radioastronomy and the third-largest identified in space. This finding sheds light on a crucial astrochemical question: the origins and pathways of carbon, the essential building block of life, in the universe and our solar system.

Researchers discovered a new, complex molecule (1-cyano pyrene), an organic molecule made up of multiple fused benzene rings and belongs to a class of compounds known as Polycyclic Aromatic Hydrocarbons (PAHs), in Taurus Molecular Cloud-1 (TMC-1), a cold interstellar cloud. TMC-1 is a region that has not yet started forming stars in the Taurus constellation, with temperatures only about 10 degrees above absolute zero.

Brett McGuire, an Assistant Professor of Chemistry at MIT and an adjunct astronomer at the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO), said, “TMC-1 is a natural laboratory for studying these molecules that go on to form the building blocks of stars and planets. These are the largest molecules we’ve found in TMC-1 to date. This discovery pushes the boundaries of our understanding of the complexity of molecules that can exist in interstellar space.”

The discovery was made using the NSF NRAO Green Bank Telescope, the world’s largest fully steerable radio telescope.

Every molecule has a unique rotational spectrum, like a fingerprint, which helps identify it. However, some polycyclic aromatic hydrocarbons (PAHs) can be challenging to detect due to their large size and lack of a permanent dipole moment. The recent observation of a cyanopyrene isomer offers indirect evidence for even larger and more complex molecules in future studies.

Identifying the unique rotational spectrum of 1-cyanopyrene required an interdisciplinary scientific team.

Astronomers study polycyclic aromatic hydrocarbons (PAHs) to understand their lifecycle and learn how they interact with the interstellar medium (ISM) and surrounding celestial bodies.

PAHs are thought to cause the unidentified infrared bands seen in many astronomical objects, which result from the infrared fluorescence of PAHs after they absorb ultraviolet (UV) photons from stars. The intensity of these bands suggests that PAHs could make up a significant portion of the carbon in the ISM.

PAHs are also found much closer to Earth. Last December, researchers analyzing samples returned from an asteroid discovered not only PAHs but also significant amounts of pyrene and naphthalene, which had previously been detected in TMC-1.

Additionally, by examining the signatures of carbon-13, a rare isotope of carbon, they determined that these PAHs likely formed at very low temperatures—similar to the 10 K found in TMC-1!

McGuire said, “This is hinting that these PAHs we’re finding in our own Solar System may have formed long before our star, in the cold dark cloud of gas and dust like TMC-1 that gave birth to the Sun. It’s wild to think we may look at the chemical archaeological record of our molecular origins in these asteroids… and at the very beginnings of that record for whatever solar systems and planets eventually form in TMC-1.”

McGuire and his colleagues plan to continue their search for other PAHs in TMC-1 to develop a more comprehensive understanding of the molecular population in this region. The detection of PAHs in this cold, dense cloud challenges traditional views on their formation and suggests new implications for their role in interstellar chemistry and astrobiology.

PAHs are considered precursors to molecules vital for the origin of life. Future research will shed light on the processes of star and planet formation and help us understand the conditions that could lead to the emergence of life in our universe.

Journal Reference:

Gabi WenZel, Ilsa Cooke, P. Bryan Changgala et al. Detection of interstellar 1-cyanopyrene: A four-ring polycyclic aromatic hydrocarbon. Science. DOI: 10.1126/science.adq6391