That's a dangerous question! Last time somebody asked it seriously, they demoted Pluto! A planet is any object orbiting the Sun which is heavy enough to both squeeze itself into a spherical shape and clear the space around its own orbit (in other words, its gravity must be powerful enough that anything nearby gets drawn in and either smashes into the planet or gets catapulted away into space). By this definition, we have eight planets in our Solar System. It's confusing, though, because we have also discovered hundreds of "planets" around other stars which are not the Sun and so don't fit the formal definition either.
The original meaning of the word planet was simply "wanderer", referring to five bright stars which moved across the sky from night to night, and which later turned out to be Mercury, Venus, Mars, Jupiter and Saturn.
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