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For something to be a planet it has to fit under a certain definition                            A planet in our solar system must:                                                                    be in orbit around the sun, have sufficient mass to assume hydrostatic equilibrium (a nearly round shape), have cleared neighborhood around its orbit,meaning no large of debris lie in its way                                                                                                                                                                                  only 8 planets in our  solar system fit this description.There used to be nine planets(including pluto)but since astronomers revised the definition of planet pluto does not meet the last requirement and its not longer to be a true planet.