With the present surge of interest in astrobiology and its emergence as a new scientific discipline in its own right, the role of a celebrated pioneer is all too often forgotten. There can be little doubt that the late Sir Fred Hoyle played a key part in relating astronomical phenomena to questions of life. One of his first contributions in this area was his introduction of the so-called anthropic principle to astronomy. By the late 1940's astronomers had worked out how the simplest chemical element Hydrogen could be converted into Helium in stars, thus providing the main energy source by which stars shine. The building of nuclei beyond Helium by stellar nuclear processes appeared difficult at the time because of instabilities in nuclei with atomic masses 5 and 8. Hoyle had the grand vision of making most if not all of the elements in the Periodic Table in stars. In the early 1950's Hoyle argued that by the very fact of our existence, the existence of life, the element Carbon had to be synthesised in quantity in stars. This could not happen, Hoyle concluded, unless the nucleus of Carbon possessed an energy level corresponding to a hitherto unknown excited state which he was able to calculate. This was necessary so that three Helium nuclei could combine first to form a Carbon nucleus in the excited state that subsequently decayed into the ground state. One of the major triumphs of Hoyle's Anthropic Principle was that his predicted excited state was subsequently discovered in the laboratory by Ward Whaling and Willy Fowler at Caltech. This discovery opened the door to a brand new discipline of Nuclear Astrophysics. In a seminal paper published in 1957, Hoyle together with Willy Fowler, Geoffrey and Margaret Burbidge showed that all the chemical elements needed for life C, N, O, P, Mg, Fe, S … were made in stars. In a sense Hoyle's work in 1957 already provided the foundation stone for astrobiology. He showed that in essence we were made of stardust.

In this work we started from the basic idea that the pure polycyclic aromatic hydrocarbons (PAHs) cannot be the real carriers of the unidentified infrared bands (UIBs), the emission spectra coming from a large variety of astronomical objects. Instead we propose a new model taken from petroleum chemistry which, we can show, is able to match both the UIBs and even the protoplanetary nebulae (PPNe) spectra. PAHs such as phenanthrene, benzoperylene, coronene and pentacene, are too pure and too specific to really exist in the interstellar medium. Instead our model proposes that the carrier of UIBs and PPNe are complex molecular mixtures like those obtained as fractions during the petroleum refining processes. These molecular mixtures are so complex that practically the investigators did not try to identify each individual component but characterized the mixture with an average molecular structure that takes into account both the average molecular weight and the average content of aromatic, naphtenic (cycloaliphatic) and aliphatic (paraffinic) fraction. We show by infrared spectroscopy that petroleum fractions obtained at certain steps of the refining process are able to match the UIBs and the PPNe infrared bands with the advantage of not being so specific as PAHs are. Namely we have used as samples a distillate aromatic extract (DAE) a treated residual aromatic extract (T-RAE) and finally a naphtenic oil. Among the three samples examined, the DAE sample was the best in matching the UIBs and PPNe spectra.

NASA Ames Research Center, Moffett Field, California, USA 7–11 April 2002