The discovery of numerous planets orbiting solar-type stars has given new currency to a question that is as old as the hills: are there other inhabited worlds? This question was posed by the Greek philosophers Democritus and Epicurus, and by the Roman poet Lucretius. In the 16th century, the idea of extraterrestrial life was taken up by Giordano Bruno, who paid with his life for his numerous iconoclastic views, but it continued to gain ground among many astronomers and philosophers such as Kepler, Kant, Huygens and Fontenelle. Thoughts initially turned to searching for traces of life in the Solar System itself, in particular on the planet Mars. In parallel with this, the first conceptions of a chemical evolution of life appeared around the 1920s, when the Russian chemist Oparin first advanced the theory that complex organic molecules could evolve, through a long series of chemical transformations, into micro-organisms. An essential step was taken in 1953, when the American chemists Miller and Urey first succeeded in the laboratory in synthesizing amino acids from plausible prebiotic conditions. From the 1970s, the discovery of more and more, and increasingly complex interstellar molecules proved that a prebiotic chemistry did indeed exist in the Universe. After the discovery of prebi-otic molecules in Titan's atmosphere at the beginning of the 1980s, it seemed that a rich prebiotic chemistry was active in the most varied types of environment, from the interstellar medium to circumstellar envelopes and to planetary atmospheres.
And now, with the discovery of many exoplanets orbiting stars near to the Sun, and comparable with it, the debate over the possibility of extraterrestrial life takes on a new dimension.
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