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Humans are carbon-based life forms, but Earth is not a carbon-based planet. Some scientists think such planets may exist, though, in orbit around distant stars. Cutaway views show a carbon-based planet, possibly made mostly of silicon carbide, or carborundum, compared with the preponderance of oxygen-rich compounds in a silicate planet like Earth. At left is an artist's view of a carbon-based planet covered in tar, where a meteorite impact has exposed a layer of diamond. Credit: Illustration by Lynette Cook

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