Has graphene been detected in space?
Graphene is a flat sheet of carbon atoms, one atom thick, that has extraordinary strength, conductivity, elasticity, and thinness.
August 11, 2011
A team of astronomers using the Spitzer Space Telescope has reported the first extragalactic detection of the C70 fullerene molecule, and the possible detection of planar C24 (a piece of graphene) in space. Letizia Stanghellini and Richard Shaw from the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, describe how collisional shocks powered by the winds from old stars in planetary nebulae could be responsible for the formation of fullerenes (C60 and C70) and graphene (planar C24).
Artist’s impression of the graphenes and fullerenes found in a planetary nebula. The detection of graphenes and fullerenes around old stars as common as our Sun suggests that these molecules and other allotropic forms of carbon may be widespread in space. Image credits: IAC; original image of the Helix Nebula: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner, STScI, & T.A. Rector, NRAO.
Planetary nebulae originate from stars similar to our Sun that have reached the end of their lives and are shedding shells of gas into space. In this case, the planetary nebulae are located in the Magellanic Clouds, two satellite galaxies to our own Milky Way that are best seen from the Southern Hemisphere. At the distance of the Magellanic Clouds, planetary nebulae appear as small fuzzy blobs.
However, unlike planetaries in our own Milky Way Galaxy whose distances are uncertain, the distance to planetaries in the Magellanic Clouds can be determined to better than 5 percent. With such accurate distances, the research team determined the true luminosity of the stars, and confirmed that the objects are, indeed, planetary nebulae and not some other object in the astrophysical zoo.
Fullerenes, or Buckyballs, are known from laboratory work on Earth and have many interesting and important properties. Fullerenes consist of carbon atoms arranged in a 3-D sphere similar to the geodesic domes popularized by Buckminster Fuller. The C70 fullerene can be compared with a rugby ball while C60 is compared to a soccer ball. Both of these molecules have been detected in the sample. Graphene (planar C24) is a flat sheet of carbon atoms, one atom thick, that has extraordinary strength, conductivity, elasticity, and thinness. Cited as the thinnest substance known, graphene was first synthesized in the lab in 2004 by Geim and Novoselov, for which they received the 2010 Nobel Prize in physics. “If confirmed with laboratory spectroscopy — something that is almost impossible with the present techniques — this would be the first detection of graphene in space,” said García-Hernández from NOAO.
The team has proposed that fullerenes and graphene are formed from the shock-induced (i.e., grain-grain collisions) destruction of hydrogenated amorphous carbon grains (HACs). Such collisions are expected in the stellar winds emanating from planetary nebulae, and this team sees evidence for strong stellar winds in the ultraviolet spectra of these stars. “What is particularly surprising is that the existence of these molecules does not depend on the stellar temperature, but on the strength of the wind shocks,” said Stanghellini.
The Small Magellanic Cloud is particularly poor in metals, but this sort of environment favors the evolution of carbon-rich planetary nebulae, which turns out to be a favorable place for complex carbon molecules. The challenge has been to extract the evidence for graphene from Spitzer data. “The Spitzer Space Telescope has been amazingly important for studying complex organic molecules in stellar environments,” said Stanghellini. “We are now at the stage of not only detecting fullerenes and other molecules, but also starting to understand how they form and evolve in stars."
“We are planning ground-based follow-up through the NOAO system of telescopes, said Shaw. “We hope to find other molecules in planetary nebulae where fullerene has been detected to test some physical processes that might help us understand the biochemistry of life.”