To date, most known extrasolar planets are gas giants like Jupiter or Neptune. Lacking even a solid surface, they are worlds very unlike our own planet. A true Earth twin must satisfy five characteristics. So far, astronomers have found only one planet with a measured size and mass similar to Earth's: Kepler 78 b, announced last October. A recent study found that most planets smaller than one and a half times Earth's diameter probably have rocky surfaces. Kepler has found more than 1,000 candidates, but they are not yet confirmed as Earth-like.
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