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IF EXTRATERRESTRIAL life were to exist, it would need a planet on which to evolve. All but one of 200-or-so planets outside the solar system discovered by astronomers so far would be quite unsuitable. That is because these planets are composed of gas. Yet the one whose discovery was announced in the early hours of Wednesday April 25th is different. Astronomers think it is rocky, like the Earth . . . .
The planet in question orbits a star called Gliese 581 that lies a mere 20 light years away in the constellation Libra . . . . Gliese 581 is a red dwarf, so-called because the star is small and the fusion reaction proceeds slowly, creating a dim glow. Nevertheless, because the planet is much closer to its star than the Earth is to the sun, it lies in what astronomers call the "habitable zone" . . . .
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