As early as the third century BC the Greek philosopher Epicurus (341-270 BC) affirmed the existence of an infinite number of worlds. In a letter to Herodotus he wrote: 'Epicurus to Herodotus, greetings... The Universe is infinite. Moreover, there is an infinite number of worlds, some like this world, others unlike it. Also, atoms are infinite in number, as has just been proved, and they are carried ever further in their courses. Mow, the atoms from which a worid might be made, or from which a worid is already made, have not all been expended on only ope worid, or on a finite number of worlds, whether they be like or unlike each other. Hence there is nothing to oppose the idea of an infinity of worlds.'
The Orion Nebula — a site of star formation - imaged by the Hubble Space Telescope in 1991.
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