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The Laplace resonance among orbiting bodies is a series of two consecutive 2:1 mean motion resonances, making a 4:2:1 period ratio among three bodies in orbit. The Galilean satellites Io, Europa, and Ganymede are in the Laplace resonance. This commensurability was first pointed out by Pierre-Simon Laplace, and now bears his name. This interaction prevents the orbits of the satellites from becoming perfectly circular (due to tidal interactions with Jupiter), and therefore permits tidal heating of Io and Europa. Recently, an extrasolar planetary system, Gliese 876, was found to contain a Laplace resonance among three orbiting planets.