Yours is a very good question concerning recent "changes" in our solar system. The new classification of Pluto has surprised and upset many people throughout the world.
In August 2006, the International Astronomical Union (IAU) formalized the definition of the term "planet." The three critera that they require of a planet are:
(1) The object must be in orbit around the Sun
(2) The object must be massive enough that its gravity pulls it into a stable spherical shape
(3) The object must have cleared the neighborhood around its orbit
By "clearing the neighborhood," they mean that the object's gravity has either attracted smaller nearby objects to it or has caused them to move to another orbit.
Criterion #3 is where Pluto fails to make the grade as a planet. It has since been reclassified as a "dwarf planet," along with Ceres and Eris.
Remember, though: Whether we call it a planet or a dwarf planet, it's still Pluto and it's still part of our solar system. As William Shakespeare wrote in Romeo and Juliet: "What's in a name? That which we call a rose by any other word would smell as sweet."
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