Searching for superEarths

Mass and radius may help identify places like home

ST. LOUIS — In their quest to find some place just like home, astronomers are exploring a new family of planets beyond the solar system — orbs only a few times heavier than Earth. Over the past three years, researchers have discovered five superEarths, ranging from five to about 10 times the mass of the Earth. With exoplanet-seeking missions like the European Space Agency’s COROT, launched in 2006, and NASA’s Kepler, set for launch later this year, astronomers expect to find many more — and much smaller — superEarths. But because the bodies are too small and faint for state-of-the-art telescopes to image, researchers have only limited information — typically only mass and radius.

So Diana Valencia of HarvardUniversity and her colleagues set out to develop criteria, using information only on mass and radius, that could distinguish a primarily wet superEarth from a dry, rocky one. (Although covered by a thin layer of water over much of its surface, Earth is considered a rocky planet by volume.) Determining which superEarths are oceanic or icy — like a scaled-up version of Jupiter’s moon Europa — and which are rocky could be crucial because life is more likely to gain a foothold on rocky orbs like ours, theory suggests. If astronomers could differentiate, they might choose to focus more of their initial studies on planets they know to be rocky. But making the distinction isn’t easy, Valencia noted in St. Louis at the April meeting of the American Physical Society.

Planets of the same mass can be roughly the same size even if they have different mixtures of icy and rocky material in the core and mantle and on the surface. Despite this difficulty, Valencia and her colleagues say they can crudely differentiate oceanic planets from rocky superEarths — if the radius of the exoplanet is measured to an accuracy of better than 5 percent and its mass to an accuracy better than 10 percent. That’s because for a given mass, a completely rocky planet cannot exceed a particular radius.

The only way it could be any larger than that maximum threshold is if at least 10 percent of the body were made of water rather than rock, and that would make the body at least partly oceanic. Theorist Sara Seager of MIT says she admires the work but notes that the model would have difficulty telling apart a rocky planet with even a small atmosphere from an oceanic planet, if astronomers only have information on mass and radius. Valencia also reported the results of another study, which indicates that bigger is better when it comes to the habitability of rocky planets. Her team found that rocky superEarths are more likely to undergo tectonic plate activity than lighter-weight ones.

The larger internal heat source of a heavier planet would more easily drive such activity, in which thin plates of material at or near the planet’s surface collide to build mountains and move continents. Plate tectonics is essential for life as we know it, because such activity recycles carbon dioxide between rock and the atmosphere, Valencia says. The carbon recycling acts as a thermostat, stabilizing the planet’s temperature.

FAR, FAR AWAY An artist’s depiction of a superEarth near its parent star. The superEarth is shown with its own moon. D. Aguilar/Harvard-Smithsonian

Therefore, a heavier superEarth, with enhanced plate tectonic activity, may offer an even more hospitable environment for life than our planet, says Valencia.

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