Here's why:
* Definition of a planet: The International Astronomical Union (IAU) defines a planet as a celestial body that:
* is in orbit around the Sun
* has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
* has cleared the neighbourhood around its orbit
* Exoplanets: We are constantly discovering new exoplanets (planets outside our solar system) and these discoveries reveal that solar systems can be very diverse. Some have more planets than ours, while others have fewer.
