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This is an artist's conception of Pluto and one of its moons, Charon. Pluto is in the background and Charon is in the foreground. Pat Rawlings, Science Applications International, NASA.


Pluto was once called the ninth major planet of our solar system. Over the last few decades, new instruments and observations have changed our understanding of Pluto. Pluto is now thought to be just a large icy body in the outer region of the solar system (called the Kuiper Belt). Astronomers say that there are thousands of icy objects that measure 100 km across or more in the Kuiper Belt. Pluto is now thought to be one of the larger icy Kuiper Belt objects. It is now called a dwarf planet, not a major planet. (see definition of "dwarf planet.)