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In the sky with diamonds? A so-called Super-Earth, planet 55 Cancri e was believed to be the first known planet to consist largely of diamond, due in part to the high carbon-to-oxygen ratio of its host star. (Artist's concept: Haven Giguere/Yale University) In the sky with diamonds? A so-called Super-Earth, planet 55 Cancri e was believed to be the first known planet to consist largely of diamond, due in part to the high carbon-to-oxygen ratio of its host star.