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Part of Hall of the Universe.
In our solar system, planets are the major bodies orbiting the Sun. Because we cannot yet observe other planetary systems in similar detail, a universal definition of a planet has not emerged. In general, planets are massive enough for their gravity to make them spherical, but small enough to avoid nuclear fusion in their cores.
The more than sixty moons of the solar system are a diverse collection of worlds. Two are larger than the planet Mercury. Some have ancient cratered surfaces, while others are geologically active. Many smaller moons appear to be asteroids or comets captured by the gravity of their planet.