Could extrasolar planets consist of graphite and diamond?

Earth-like planets around other stars may be composed not of stone but of carbon, with a graphite crust, diamond interior and tar oceans.
Lynette Cook

Astronomy is the science of the exotic, but the thing that astronomers most want to find is the familiar: another planet like Earth, a hospitable face in a hostile cosmos. The Kepler spacecraft, which was launched last March, is their best instrument yet for discovering Earth-like planets around sunlike stars, as opposed to the giant planets that have been planet finders’ main harvest so far. Many predict that 2010 will be the year of exo-Earths. But if the giant planets, which looked nothing like what astronomers had expected, are any indication, those Earths may not be so reassuringly familiar either.

It has dawned on theorists in recent years that other Earth-mass planets may be enormous water droplets, balls of nitrogen or lumps of iron. Name your favorite element or compound, and someone has imagined a planet made of it. The spectrum of possibilities depends largely on the ratio of carbon to oxygen. After hydrogen and helium, these are the most common elements in the universe, and in an embryonic planetary system they pair off to create carbon monoxide. The element that is in slight excess ends up dominating the planet’s chemistry.

In our solar system, oxygen dominates. Although we tend to think of our planet as defined by carbon, the basis of life, the element is actually a fairly minor constituent. The terrestrial planets are made of silicate minerals, which are oxygen-rich. The outer solar system abounds in another oxygen-rich compound, water.

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