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Carbon planets may be rich in diamonds but waterless

Jenny Winder, News Writer
Oct 27, 2013, 7:00 UTC

Sen—According to NASA-funded theoretical research, carbon rich planets, including so-called diamond planets, may lack oceans.

Because our Sun is a carbon-poor star, Earth is made up largely of silicates, not carbon. Stars with more carbon than the sun are predicted to make planets largely made of carbon, perhaps even with layers of diamond.

Scientists modeling the ingredients in these carbon-based planetary systems determined they lack icy water reservoirs thought to supply planets with oceans.

"The building blocks that went into making our oceans are the icy asteroids and comets," said Torrence Johnson of NASA's Jet Propulsion Laboratory in Pasadena, California. "If we keep track of these building blocks, we find that planets around carbon-rich stars come up dry," he said.

Johnson and his colleagues say the extra carbon in developing star systems would snag the oxygen, preventing it from forming water.

"It's ironic that if carbon, the main element of life, becomes too abundant, it will steal away the oxygen that would have made water, the solvent essential to life as we know it," said Jonathan Lunine of Cornell University, a collaborator on the research.

One of the big questions in the study of planets beyond our Solar System is whether or not they are habitable. NASA's Kepler mission has found several planets within the habitable zone, where temperatures are warm enough for water to pool on the surface, and researchers continue to scrutinize the Kepler data for candidates as small as Earth.

Even if a planet is found in the "Goldilocks" zone, where oceans could, in theory, abound, is there actually enough water available to wet the surface? Johnson and his team addressed this question with planetary models based on measurements of our Sun's carbon-to-oxygen ratio. Our Sun, like other stars, inherited a soup of elements from the Big Bang and from previous generations of stars, including hydrogen, helium, nitrogen, silicon, carbon and oxygen.

Models accurately predict how much water was locked up in the form of ice early in the history of our Solar System, before making its way to Earth. Comets and/or the parent bodies of asteroids are thought to have been the main water suppliers, though researchers still debate their roles. Either way, the objects are said to have begun their journey from far beyond Earth, past a boundary called the "snow line," before impacting Earth and depositing water deep in the planet and on its surface. When the researchers applied the planetary models to the carbon-rich stars, the water disappeared. "There's no snow beyond the snow line," said Johnson.

"All rocky planets aren't created equal," said Lunine. "So-called diamond planets the size of Earth, if they exist, will look totally alien to us: lifeless, ocean-less desert worlds."