The newfound world, dubbed Kepler-22b, is much bigger than our planet - but, even so, it's also the smallest world yet discovered outside our Solar System, and finding it does bode well for the eventual detection of a true twin, a planet that really is a second Earth.

BBC: Viewpoint: The obsession with a 'twin Earth'

Since the very first planets outside our solar system were discovered in the early 1990s, the hope has been to find an "Earth twin" - a planet like ours, orbiting a star like ours, at a distance like ours.

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And it's not even that Pluto's twin, Charon, is going to be a planet too.

NPR: A Down-to-Earth View of the Planet Debate

Think Star Wars with its Force or the sword-and-planet genre with its fallen civilizations and its twin reliance on magic and tech.

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Their simulations included the effects of eclipses and tidal heating on climate for an earth-twin in orbit around a Jupiter-like gaseous giant planet.

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Opportunity landed on the Red Planet in January 2004, about three weeks after its "twin", the Spirit rover.

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