What Makes a Planet?

Greg Laden draws our attention to an object named Vesta, which by itself makes up 9% of the asteroid belt. Greg says "if you take the largest handful of objects in the asteroid belt, Ceres, Vesta, Pallas and 10 Hygiea, you've got half of the mass of the entire thing, according to the most current estimates." According to NASA, Vesta is even differentiated, meaning it was once hot enough to form a core, mantle, and crust. On Life at the SETI Institute, the Analysis Lead on NASA's Kepler project explains how to spot a planet from hundreds of millions of miles away. Dr. Jon Jenkins says "We're looking for one part per 10,000 drop in brightness caused by this tiny planet blocking a small fraction of the light from the star." Kepler finds about ten new planetary candidates every day, and can also "hear" starquakes, the "songs of the stars." Finally, on Starts With a Bang!, Ethan Siegel brings planetary dynamics closer to home. He says earthquakes occur as the planet differentiates itself, bringing the heaviest elements to the core, and the lightest elements to the surface. Every time this happens, the world spins a little faster.

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