A question as old as astronomy itself

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From Democritus and Epicurus to Aristotle and Seneca, Greek and then Roman philosophers often aired the question of the plurality of worlds. For Lucretius, 'there are, in other regions of space, Earths other than ours, different races of men, and different wild creatures.' Diogenes Laertius wrote: 'The Universe is infinite... of it, one part is a plenum, and the other a vacuum. He [Leucippus] also says that the elements, and the worlds which are derived from them, are infinite, and are dissolved again into them.'

More than a millennium later the debate took on a new dimension with the rise of the Copernican system, which taught that the planets revolve around the Sun. No longer was the Earth at the centre of everything. The question of the plurality of worlds was restated: if the Sun is but one star among all those strewn across the night sky, could not they also possess their retinues of planets? And might some of those distant worlds harbour life? At the beginning of the seventeenth century, Giordano Bruno was among the first to put forward the hypothesis: 'If the Universe be infinite, it must be that there exists a plurality of Suns... Around those Suns there may revolve worlds greater or smaller than our own.' This idea - considered heretical at the time - cost Bruno his life (although some of his other opinions and beliefs contributed to his demise). More than a century later, Christiaan Huygens was the first person to attempt direct observations of exoplanets; but he soon realised that this was impossible with the instruments then available. In the seventeenth century, philosophers and writers wondered about the possibility of inhabited worlds nearer to ours: might there be life elsewhere within the solar system? In the works of Fontenelle and Cyrano de Bergerac, the likelihood of living things upon Venus and the Moon was explored. These may have been rather vain speculations, but two centuries later the debate came once more to the fore with Giovanni Schiaparelli's announcement of the existence of canali on Mars -taken by some to be proof of an intelligent civilisation. Astronomical opinion remained divided on this question for a long time, until space probes settled it once and for all in the 1960s.

Cyrano de Bergerac on the Moon. Illustration from Les Aventures Prodigieuses de Cyrano de Bergerac, text and illustrations by Henriot (Epinal, c.1905).

2.1 The plurality of inhabited worlds 15

2.1 The plurality of inhabited worlds 15

'They leapt onto the rings of Saturn.' Illustration from Micromegas, by Voltaire.

(Photogravure from 1867.)

However, spaceborne exploration of the Red Planet has, during the last two decades, firmly established that water once flowed there. The search for fossil life on Mars is still very much on the minds of astronomers.

The twentieth century saw further important steps in this field, with new observational methods allowing a dedicated search for the companions of nearby stars. Piet van de Kamp announced his discovery of a planet orbiting Barnard's Star, but methods available in the 1950s were not precise enough to allow confirmation of this.

Meanwhile, Stanley Miller's historic experiment in 1953 was a springboard for studies in prebiotic chemistry - the chemistry of the development of molecules necessary for the emergence of life. Echoing the work of the biochemist Oparin -who in 1924 had stated that life is the product of a long evolutionary process involving ever more complex organic chemistry - Miller was the first to synthesise amino acids from a gaseous mixture of water, methane and ammonia, into which electrical discharges were introduced. Many later experiments carried out under similar conditions have given rise to amino acids and nucleotides: the 'building blocks of life' involved in the synthesis of DNA.

Another surprise awaited researchers. Certain meteorites were found to contain traces of various prebiotic molecules and amino acids. Could life have been delivered to Earth on the asteroidal and cometary (meteoritic) material which bombarded it? And if this happened, might it not also have occurred on other distant planets? We now know that prebiotic molecules are abundant in comets and in the atmosphere of Saturn's satellite Titan, and are also found in the interstellar medium and in the environs of stars. Exobiology - the study of extraterrestrial life - is now undergoing a veritable revolution, driven by the discovery of exoplanets and by planetary exploration.

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