“The amazing thing about these observations, about this discovery, and about these molecules, is that no one had looked, or looked hard enough,” said Michael McCarthy, an astrochemist and Acting Deputy Director of Harvard Center for Astrophysics (CfA) about the discovery of a vast, previously unknown reservoir of new molecules in a cold, dark molecular cloud in the interstellar medium for the first time, “It makes you wonder what else is out there that we just haven’t looked for.”
“These molecules represent a vast reservoir of elemental carbon. Many of the molecules we consider to be possibly ‘biologically relevant’ – things such as glycolaldehyde and formamide – have carbon as the foundation of their structures, wrote Brett McGuire, Assistant Professor of Chemistry at the Massachusetts Institute of Technology, and the Project Principal Investigator for GOTHAM, using Green Bank Telescope (GBT) data, in an email to The Daily Galaxy. “
“Thus, it is important for us to be able to ask: are the PAHs (polycyclic aromatic hydrocarbon) a carbon sink or a potential source of reactive carbon?” added McGuire. “That is to say: are we building PAHs and steadily removing carbon from the chemical reaction networks that build smaller prebiotic molecules, or are these PAHs reacting and being broken down, serving as a feedstock for reactions generating prebiotic molecules? This is something we can now start to explore in more detail now that we have the ability to detect and study individual PAH molecules and their reactions.”
Tracking the PAHs
“It is all about being able to track this reservoir of carbon. How many of these PAHs survive the formation of a star? Are they incorporated into small rocky/icy bodies like comets and meteorites to rain down reactive organic carbon on the surfaces of planets?” continued McGuire in his email ” Do they get broken down by the radiation of the new star, driving further carbon chemistry in this phase of star/planet formation? These are harder questions to answer – as it will be substantially harder to observe these molecules in sources where stars are being born. But, if we can build models based on our observations of these species early in the process, we can make predictions about how their chemistry will evolve alongside the changing physical conditions of the forming planetary system, even if we cannot observe that evolution directly.”
200 Types of Molecules Known Floating in Space –Until Now
The compounds that are found naturally on Earth such as water (H2O) and carbon dioxide (CO2)—that make up the huge diversity of materials on this planet—are just a fraction of those discovered so far in the cosmos. Scientists have found some 200 types of molecules floating in space. In 2015, astronomers studying spectroscopic data –light broken down into their constituent wavelengths–displayed on screen from Hubble’s iconic Horsehead Nebula revealed the chemical makeup of the nebula that looked like blips on a heart monitor, with each wiggle indicating that some molecule had emitted light of a particular wavelength. What the researchers saw displayed was a mystery—several small unidentified lines of a molecule completely unknown to science.
Every molecule has its own unique wiggles based on the orientation of its protons, neutrons and electrons. Most of the wiggles in the Horsehead data, reported Clara Moskowitz for Scientific American in The Hunt for Alien Molecules, were easily attributable to common chemicals such as carbon monoxide, formaldehyde and neutral carbon. But one spot in the Horsehead had several small unidentified lines equally separated from one another in frequency– an enigma.
The Horsehead Nebula,” writes Moskowitz , “is no aberration. Almost everywhere in the universe astronomers look—if they peer closely enough—they see unidentified spectral lines. The compounds we humans are familiar with, the species responsible for the huge diversity of materials on this planet, are just a fraction of those nature has created.
Answers to a Three-Decades-old Scientific Mystery
The new discoveries announced by the Harvard CfA, made by detecting individual polycyclic aromatic hydrocarbon molecules, are beginning to answer a three-decades-old scientific mystery: how and where are these molecules formed in space?
“We had always thought polycyclic aromatic hydrocarbons were primarily formed in the atmospheres of dying stars,” said McGuire. “In this study, we found them in cold, dark clouds where stars haven’t even started forming yet.”
Aromatic molecules, and PAHs–shorthand for polycyclic aromatic hydrocarbons–are well known to scientists. Aromatic molecules exist in the chemical makeup of human beings and other animals, and are found in food and medicines. As well, PAHs are pollutants formed from the burning of many fossil fuels and are even amongst the carcinogens formed when vegetables and meat are charred at high temperatures.
PAHs are considered as precursors to the formation of molecular clouds—the so-called “molecular factories” of more complex organic molecules that can include the precursors to life as we know it—This could open up new models of how carbon-containing material in deep space and in the rich atmospheres of planets and their moons in our solar system evolve and originate.
“Polycyclic aromatic hydrocarbons are thought to contain as much as 25-percent of the carbon in the universe,” said McGuire, who is also a research associate at the Center for Astrophysics. “Now, for the first time, we have a direct window into their chemistry that will let us study in detail how this massive reservoir of carbon reacts and evolves through the process of forming stars and planets.”
“What They Found was Astonishing”–Zooming in on The Taurus Molecular Cloud
Scientists have suspected the presence of PAHs in space since the 1980s but the new research, detailed in nine papers published over the past seven months, provides the first definitive proof of their existence in molecular clouds. To search out the elusive molecules, the team focused the 100m behemoth radio astronomy GBT on the Taurus Molecular Cloud, or TMC-1–a large, pre-stellar cloud of dust and gas located roughly 450 light-years from Earth that will someday collapse in on itself to form stars–and what they found was astonishing: not only were the accepted scientific models incorrect, but there was a lot more going on in TMC-1 than the team could have imagined.
These dark clouds are the initial birthplaces of stars and planets. So, these previously invisible aromatic molecules will also need to be thought about at each later step along the way to the creation of stars, planets, and solar systems like our own.
“From decades of previous modeling, we believed that we had a fairly good understanding of the chemistry of molecular clouds,” said McCarthy, whose research group at the CfA made the precise laboratory measurements that enabled many of these astronomical detections to be established with confidence. “What these new astronomical observations show is these molecules are not only present in molecular clouds, but at quantities which are orders of magnitude higher than standard models predict.”
Radio Astronomy Reveals Individual Molecules
“For the last 30 years or so,” said McGuire, about previous studies revealed only that there were PAH molecules out there, but not which specific ones, “scientists have been observing the bulk signature of these molecules in our galaxy and other galaxies in the infrared, but we couldn’t see which individual molecules made up that mass. With the addition of radio astronomy, instead of seeing this large mass that we can’t distinguish, we’re seeing individual molecules.”
Much to their surprise, the team didn’t discover just one new molecule hiding out in TMC-1. Detailed in multiple papers, the team observed 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans-(E)-cyanovinylacetylene, HC4NC, and propargylcyanide, among others.
Discovered a Giant Warehouse of Molecules and Chemistry
It’s like going into a boutique shop and just browsing the inventory on the front-end without ever knowing there was a back room. We’ve been collecting little molecules for 50 years or so and now we have discovered there’s a back door. When we opened that door and looked in, we found this giant warehouse of molecules and chemistry that we did not expect,” said McGuire. “There it was, all the time, lurking just beyond where we had looked before.
McGuire and other scientists at the GOTHAM project have been “hunting” for molecules in TMC-1 for more than two years, following McGuire’s initial detection of benzonitrile in 2018. The results of the project’s latest observations may have ramifications in astrophysics for years to come.
Unlike Anything We’ve Previously Been Able to Detect
“We’ve stumbled onto a whole new set of molecules unlike anything we’ve previously been able to detect, and that is going to completely change our understanding of how these molecules interact with each other. It has downstream ramifications,” said McGuire, adding that eventually these molecules grow large enough that they begin to aggregate into the seeds of interstellar dust. “When these molecules get big enough that they’re the seeds of interstellar dust, these have the possibility then to affect the composition of asteroids, comets, and planets, the surfaces on which ices form, and perhaps in turn even the locations where planets form within star systems.”
The discovery of new molecules in TMC-1 also has implications for astrochemistry, and while the team doesn’t yet have all of the answers, the ramifications here, too, will last for decades.
Tip of the Iceberg
“We’ve gone from one-dimensional carbon chemistry, which is very easy to detect, to real organic chemistry in space in the sense that the newly discovered molecules are ones that a chemist knows and recognizes, and can produce on Earth,” said McCarthy. “And this is just the tip of the iceberg. Whether these organic molecules were synthesized there or transported there, they exist, and that knowledge alone is a fundamental advance in the field.”
Before the launch of GOTHAM in 2018, scientists had cataloged roughly 200 individual molecules in the Milky Way’s interstellar medium. These new discoveries have prompted the team to wonder, and rightly so, what’s out there.
This new aromatic chemistry that scientists are finding isn’t isolated to TMC-1. A companion survey to GOTHAM, known as ARKHAM–A Rigorous K/Ka-Band Survey Hunting for Aromatic Molecules–recently found benzonitrile in multiple additional objects.
Pushing the Limit of Chemistry in Space
“Incredibly, we found benzonitrile in every single one of the first four objects observed by ARKHAM,” said Andrew Burkhardt, a Submillimeter Array Postdoctoral Fellow at the CfA and a co-principal investigator for GOTHAM. “This is important because while GOTHAM is pushing the limit of what chemistry we thought is possible in space, these discoveries imply that the things we learn in TMC-1 about aromatic molecules could be applied broadly to dark clouds anywhere.
Image top of the page: M. Weiss, Harvard/Smithsonian CfA