In 2004, a team of scientists discovered hydrocarbons called anthracene and pyrene in an amazing structure called the Red Rectangle!
Here two stars 2300 light years from us are spinning around each other while pumping out a huge torus of icy dust grains and hydrocarbon molecules. It’s not really shaped like a rectangle or X—it just looks that way from here. The whole scene is about one third of a light year across.
This was first time such complex molecules had been found in space:
• Uma P. Vijh, Adolf N. Witt, and Karl D. Gordon, Small polycyclic aromatic hydrocarbons in the Red Rectangle, The Astrophysical Journal 619 (2005), 368–378.
Wherever carbon-containing materials suffer incomplete combustion, you get PAHs, or polycyclic aromatic hydrocarbons. In Earth you can find them in soot, or the tarry stuff that forms in a barbecue grill. They’re common in outer space, too.
But what are PAHs like, exactly? They’re made of hexagonal rings of carbon atoms, with some hydrogens along the edges:
Benzene has a single hexagonal ring, with 6 carbons and 6 hydrogens. You’ve probably heard about naphthalene, which is used for mothballs: this consists of two hexagonal rings stuck together. True PAHs have more. With three rings you can make anthracene:
and phenanthrene:
With four, you can make napthacene:
pyrene:
triphenylene:
and chrysene:
And so on! The game just gets more complicated as you get to use more puzzle pieces.
By now, lots of organic molecules have been found in interstellar or circumstellar space. There’s a whole ‘ecology’ of organic chemicals out there, engaged in complex reactions. Life on planets might someday be seen as just an aspect of this larger ecology. PAHs are probably among the dominant players in this ecology, at least at this stage.
Indeed, I’ve read that about 10% of the interstellar carbon is in the form of PAHs—big ones, with about 50 carbons per molecule. They’re common because they’re incredibly stable. They’ve even been found riding the shock wave of a supernova explosion!
PAHs are also found in meteorites called ‘carbonaceous chondrites’. These space rocks contain just a little carbon: about 3% by weight. But 80% of this carbon is in the form of PAHs.
Here’s an interview with a scientist who thinks PAHs were important precursors of life on Earth:
• Aromatic world, interview with Pascale Ehrenfreund, Space Daily.
Also try this:
• PAH world hypothesis, Wikipedia.
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