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See Inside January 2010

Earth-Like Planets May Be Made of Carbon

Could extrasolar planets consist of graphite and diamond?



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.

A new study shows in detail how carbon lost out. Jade Bond of the University of Arizona and the Planetary Science Institute (PSI), Dante Lauretta of Arizona and David O’Brien of PSI have simulated how chemical elements got distributed around the solar system as it formed. They find that carbon remained in a gaseous state within the protoplanetary disk and was eventually blown out into deep space; the embryonic Earth wound up with none at all. The carbon in our bodies must have been delivered later, by asteroids and comets that formed under conditions that allowed them to incorporate the element.

Had the carbon-oxygen balance tilted the other way, Earth would have turned out very differently, as Marc Kuchner, then at Princeton University, and Sara Seager, then at the Carnegie Institution of Washington, argued in 2005. It would consist not of silicates but of carbon-based compounds such as silicon carbide and, indeed, pure carbon itself. The crust would be mainly graphite, and a few kilometers underground the pressures would be high enough to form a rigid shell of diamond and other crystals. Instead of water ice, the planet would have carbon monoxide or methane ice; instead of liquid water, it might have oceans of tar.

The galaxy could be filled with such worlds. According to an observational survey Bond cites, the average planet host star has a higher carbon-to-oxygen ratio than the sun does, and her team’s simulations predict that the most enriched systems give birth to carbon planets. “Some of these compositions differ greatly from solar and as a result produce terrestrial planets with vastly different compositions,” Bond says.

To be sure, other surveys have found that the sun is indistinguishable from the average star in its class. The Kepler spacecraft may help settle the question, because even the limited amount of information it can glean about planets—their mass and radius—is enough to tell their general composition.

Carbon Earths might be especially prevalent in more bizarre settings, such as the environs of white dwarfs and neutron stars. Regions of the galaxy that are rich in heavy elements generally, such as the galactic center, have higher carbon-to-oxygen ratios. As time passes and stars continue to manufacture heavy elements, the balance everywhere will tilt in favor of carbon.

These and other astronomical discoveries turn the tables on our notions of the familiar and unfamiliar. Most of the galaxy is dark matter; most suns are dimmer and redder than our sun; and now, it seems, other Earths may not be especially Earth-like. If anything departs from the norm and deserves to be called exotic, it is us.

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