There's an unidentified source of infrared throughout the universe. By looking at the specific wavelengths of the light, scientists think that come from carbon - but not just any carbon, a special kind where the atoms are arranged in multiple hexagonal rings. No one has been able to spot one of these multi-ring "polycyclic aromatic hydrocarbons," or PAHs in space - even though the infrared emissions imply that these PAHs should make up 10 per cent of the universe's carbon. Now, scientists have found a new hint.
A team of researchers in the United States and Russia are now reporting spotting a special single-carbon-ring-containing molecule, called benzonitrile, with a radio telescope in a part of space called the Taurus Molecular Cloud-1. Benzonitrile only has one hexagonal ring of carbon, so it's not a polycyclic aromatic hydrocarbon itself. But it could be a potential precursor and could help explain the mysterious radiation.
Before you even ask, yes, this "aromatic" benzonitrile molecule has a smell. "I can tell you from personal experience it smells like almonds," study first author Brett McGuire from the National Radio Astronomy Observatory told Gizmodo, who has encountered the molecule in the lab.
One of the paper's other authors, Sergei Kalenskii, compiled lots of data from different molecular spectra, the different wavelengths of light emitted from the molecule, and found hints that benzonitrile should exist in space — but didn't have a knockout observation to prove it. Some of the team members in the lab refined exactly what this molecule's rotational spectrum, the light it emits from tumbling around in space, should look like. Others then used the football field-sized Robert C. Byrd Green Bank Telescope in West Virginia to find that same spectrum in a feature called the Taurus Molecular Cloud-1. Just look at this telescope:
A limitation of the paper, published in Science today, is that benzonitrile isn't a PAH, but a precursor to a PAH (though benzonitrile might be adding to the unidentified infrared on its own right). That means scientists haven't directly detected what they think is the source of the infrared radiation — but a molecule they think they can use as a proxy.
Your next question should be, well, what makes benzonitrile, a molecule with a single carbon ring, a proxy for the multiple-ringed molecules? The carbon ring alone without any extra doodad atoms attached is called benzene, but benzene is much harder to detect with radio astronomy than benzonitrile is, said McGuire. Perhaps the scientists can use the benzonitrile as a proxy to understand how much benzene there is, and perhaps use that to gather more information about the presumed presence of PAHs.
Valentine Wakelam from the University of Bordeaux told Gizmodo that "this new detection has confirmed the presence of PAHs" in the interstellar medium. But there's still a lot to be done in order to establish what chemistry actually produces the PAHs. She points out that it's also possible that maybe PAHs turn into benzonitrile in a top-down approach - they get broken apart into these smaller units. "We have a lot to do yet," she said.
McGuire was especially excited that this was the largest molecule discovered by a radio telescope. But now it's time to take more data. "We can hopefully move from a smaller to a larger number of statistics to get an overall view to how chemistry is evolving in space."