In the bewildering quagmire that is the gas between the stars, the Hubble Space Telescope has identified evidence of ionised buckminsterfullerene, the carbon molecule known colloquially as "buckyballs".
Containing 60 carbon atoms arranged in a soccer ball shape, buckminsterfullerene (C60) occurs naturally here on Earth - in soot. But in 2010, it was also detected in a nebula; in 2012, it was found in gas orbiting a star. Now we have the strongest evidence yet that it's also floating in the interstellar medium - the sparse, tenuous gas between the stars.
"Combined with prior, ground-based observations .. our Hubble Space Telescope spectra place the detection of interstellar [buckminsterfullerene] beyond reasonable doubt," the researchers wrote in their paper.
It's difficult to directly study the interstellar medium, since it's highly diffuse. But we can see starlight shining beyond it. As that starlight travels through the interstellar medium, it slightly changes based on the composition of what it is passing through - some wavelengths are absorbed by the gas.
An instrument called a spectrograph can separate the light it detects into a spectrum, a bit like a prism. Then astronomers here on Earth interpret that spectrum, identifying the light signatures of the elements.
There are spectral features called diffuse interstellar bands that have been identified as absorption features of the interstellar medium. But, because we don't know a lot about the interstellar medium - the conditions under which it forms, for instance - it gets pretty tricky to identify the individual molecules within those bands.
Previous papers have claimed detection of buckminsterfullerene bands, but none showed evidence beyond reasonable doubt, according to a team of researchers led by physicist Martin Cordiner of NASA Goddard SFC. This is partially because of the interference produced by Earth's atmosphere when using a terrestrial telescope.
So the team turned to the Hubble Space Telescope in Earth's orbit, which conveniently eliminates that interference. They observed 11 stars, obtaining ultra-high signal-to-noise spectra of seven stars that are significantly reddened by the interstellar medium, and of four that are not.
They then studied these spectra for absorption signals at four wavelengths associated with buckminsterfullerene.
The team made reliable detections in three of the four wavelengths (the remaining wavelength was expected to return only a faint signal anyway) for the reddened stars, and none at all in the control stars. The strengths of the signals was also consistent with measurements made in a laboratory in 2018.
This, the researchers said, is the strongest evidence yet for buckminsterfullerene in the interstellar medium.
It's a lovely result. It tells us that the molecule can exist in the ghostly spaces between the stars, although we still don't know where it formed or how it got there.
It also tells us that the interstellar medium can support more massive molecules than we knew. Previously, the largest molecules definitively detected in the interstellar medium only had 3 atoms that were heavier than hydrogen, while buckyballs have 60.
"The confirmation of interstellar [buckminsterfullerene] represents a breakthrough in our understanding of chemical complexity in the diffuse interstellar medium [..] bringing a new understanding of the types of molecules that may be responsible for the remaining (unidentified) diffuse interstellar bands," the researchers wrote in their paper.
"Further high-sensitivity observations are recommended to better constrain the strengths and profiles of the weaker buckminsterfullerene bands, combined with additional laboratory and theoretical studies that may enable the exploitation of the buckminsterfullerene bands as probes of interstellar physics and chemistry."
The paper has been published in The Astrophysical Journal Letters.
This article was first published in April 2019.