A team led by researchers at MIT has detected pyrene, a complex carbon-containing molecule, in a distant interstellar cloud. This finding opens new avenues for understanding the chemical origins of our solar system. Pyrene, a type of polycyclic aromatic hydrocarbon (PAH), was found in a molecular cloud similar to the one from which our solar system formed.

This marks the first time a complex form of carbon essential for life on Earth has been observed outside the solar system. Its discovery sheds light on how the compounds necessary for life could originate in space. The team detected pyrene in
a star-forming region known as the Taurus Molecular Cloud, located 430 light-years away, making it one of the closest such clouds to Earth.

This discovery also aligns with recent findings from the asteroid Ryugu, suggesting that pyrene may have played a key role in the carbon composition of the early solar system. To learn more about the significance of this discovery, EdPublica interviewed the researchers behind the study– Gabi Wenzel, Ilsa Cooke, and Brett McGuire, who shared their insights on the implications of pyrene’s presence in space and its potential impact on our understanding of star and planet formation. Brett McGuire is an assistant professor of chemistry at MIT, Ilsa Cooke is an assistant professor of chemistry at the University of British Columbia, and Gabi Wenzel is a postdoctoral researcher in McGuire’s group at MIT.

Below, the team responds to questions from EdPublica Editor Dipin Damodharan about this unexpected and exciting discovery.

‘Pyrene could be a major source of carbon in our solar system’

Q: How does the discovery of pyrene in TMC-1 enhance our understanding of the chemical inventory that contributed to the formation of our solar system?

Gabi Wenzel:

Stars much like our own sun are born from dense molecular clouds. The discovery of pyrene in a molecular cloud called TMC-1, one that might be very similar to our sun’s natal cloud and which will go on to form a star of its own, significantly enhances our understanding of the chemical inventory that contributed to the formation of our own solar system. As a polycyclic aromatic hydrocarbon (PAH), pyrene is one of the most complex organic molecules found in early molecular clouds, suggesting that the building blocks of organic matter were available in the environments where stars and their orbiting (exo)planets form.

This discovery sheds light on the chemical processes occurring in interstellar space, including gas-phase and surface reactions on dust grains, which are crucial for the evolution of organic chemistry. This further supports the notion that the primordial materials of our solar system contained a diverse range of organic compounds, providing insights into the potential for prebiotic chemistry on a young Earth and planetesimals.

Q: What specific challenges did you face in detecting pyrene, given that it is invisible to traditional radio astronomy methods, and how did the use of cyanopyrene help overcome these challenges?

Gabi Wenzel:

Pyrene, a fully symmetric PAH, does not possess a permanent electric dipole moment and hence is invisible in radio astronomical observations or rotational spectrometers in the laboratory. The CN radical is highly abundant in the cold and dark molecular cloud TMC-1, an environment that is about 10 K cold and in which you’d assume little chemistry to happen. However, earlier experimental works have shown that the CN addition (followed by hydrogen abstraction) to ringed hydrocarbon species such as benzene and toluene at low temperatures is a barrierless process.

Adding a CN (nitrile) group to a hydrocarbon will drastically increase its permanent electric dipole moment and so allow rotational transitions. Indeed, several CN-functionalized species have been detected in TMC-1 and other sources, among which the CN-substituted benzene (cyanobenzene or benzonitrile) and other smaller PAHs, with cyanopyrene being the largest molecule found via radio astronomy to date, allowing us to infer the presence of pyrene itself.

Q: Can you elaborate on what it means for our understanding of carbon sources in the solar system that pyrene is found in both TMC-1 and asteroid Ryugu?

Ilsa Cooke:

TMC-1 is a famous example of a cold molecular cloud, one of the earliest stages of star and planet formation, while asteroids like Ryugu represent snapshots of later stages in the formation of solar systems. Asteroids are formed from material in the solar nebula that was inherited from the molecular cloud stage. Our radio observations of TMC-1 let us observe pyrene early on and possibly under conditions where it is first forming. Isotope signatures of the pyrene in Ryugu suggest it was formed in a cold interstellar cloud. From these two unique sets of measurements, we can start to unravel the inheritance of pyrene, and PAHs more generally, from their birth in interstellar space and their journey to new planets. If PAHs can survive all the way from the molecular cloud stage, they may provide planets with an important source of organic carbon.

Q: What are the different formation routes of PAHs that your research suggests, and how do these differ from previous hypotheses about PAH formation in space?

Ilsa Cooke:

Our results, combined with those of Zeichner et al., who measured pyrene in Ryugu, suggest that pyrene may form at low temperatures by “bottom-up” routes in molecular clouds. Previously, PAHs were most commonly associated with formation in high-temperature (ca. 1000 K) environments in the envelopes of dying stars. These stars are thought to eject their PAHs, along with other carbon-rich molecules, into the diffuse interstellar medium.

However, the diffuse medium is a tenuous, harsh environment permeated by ultraviolet photons, and most astrochemists think that small PAHs would not survive their journey through the diffuse medium into dense molecular clouds. So we are still left with a puzzle: does that pyrene that we observe in TMC-1 form there, or was it formed somewhere else but it was able to survive its journey more efficiently than previously thought? If the pyrene is indeed formed within TMC-1, we do not yet know the chemical mechanism. Many possibilities exist, so close collaborations between laboratory astrochemists and observers will be critical to answer this question.

Q: What are your plans for investigating larger PAH molecules in TMC-1, and what specific hypotheses are you looking to test with these investigations?

Brett McGuire:

We have a number of other targets lined up – again focusing on PAH structures that should show this special stability demonstrated by pyrene. They present the same experimental challenges, including needing to devise appropriate synthetic routes in the laboratory before collecting their spectra. The major question is just how complex the PAH inventory actually gets at this earliest stage of star formation.

Prior to our work in TMC-1, nearly everything we knew about PAHs came from infrared observations of bulk properties in much warmer and more energetic regions, where PAHs are thought to be much larger. Does the population in TMC-1 look the same as in these regions? Is it at an earlier stage of chemical evolution? And how does this distribution compare to what we see in our own Solar System?

Q: How do your findings about pyrene and PAHs in interstellar clouds influence our broader understanding of organic chemistry in the universe, particularly in relation to the origins of life?

Brett McGuire:

Life as we know it depends on carbon – it is the backbone upon which all our molecular structures are constructed. Yet, the Earth overall is somewhat depleted in carbon relative to what we’d naively expect, and we still don’t fully understand where the carbon we do have came from originally. PAHs in general seem to be a massive reservoir of reactive carbon, and what we are now seeing is that that reservoir is already present at the earliest stages of star-formation. Combined with the evidence from Ryugu, we’re now also seeing indications that the inventory of PAHs, and thus this reservoir of carbon, may actually survive from this dark molecular cloud phase through the formation of a star to be eventually incorporated into the planetary system itself.