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We present ten new Spitzer detections of fullerenes in Magellanic CloudPlanetary Nebulae, including the first extragalactic detections of the C70molecule. These new fullerene detections together with the most recentlaboratory data permit us to report an accurate determination of the C60 andC70 abundances in space. Also, we report evidence for the possible detection ofplanar C24 in some of our fullerene sources, as indicated by the detection ofvery unusual emission features coincident with the strongest transitions ofthis molecule at ~6.6, 9.8, and 20 um. The infrared spectra display a complexmix of aliphatic and aromatic species such as hydrogenated amorphous carbongrains (HACs), PAH clusters, fullerenes, and small dehydrogenated carbonclusters (possible planar C24). The coexistence of such a variety of molecularspecies supports the idea that fullerenes are formed from the decomposition ofHACs. We propose that fullerenes are formed from the destruction of HACs,possibly as a consequence of shocks driven by the fast stellar winds, which cansometimes be very strong in transition sources and young PNe. This is supportedby the fact that many of our fullerene-detected PNe show altered [NeIII]/[NeII]ratios suggestive of shocks as well as P-Cygni profiles in their UV linesindicative of recently enhanced mass loss.

The formation of fullerenes: clues from new C60, C70, and (possible) planar C24 detections in Magellanic Cloud Planetary Nebulae