The International Astronomical Union defines a planet as a celestial body that orbits the sun, is massive enough that gravity has forced it into a spherical shape, and has cleared away other objects ...
To be a planet, NASA says an object must be big enough that its gravity has cleared away any other objects of a similar size ...
The rusty world is full of mysteries—and some of the solar system's most extreme geology. Learn more about Earth's smaller, colder neighbor.
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