According to the International Astronomical Union (IAU), a planet is an astronomical body orbiting a star that is massive enough to be rounded by its own gravity but is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of any of numerous small celestial bodies that may have existed at an early stage of its development.
A rock that orbits a sun. We live on the third rock from our sun. There used to be nine planets orbiting at different distances from our sun; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. A mnemonic used to remember the order of the planets is My Very Excellent Mother Just Served Us Nice Pizza!
And then there were eight! The IAU decided in 2006 that Pluto was not a planet, but a dwarf planet. So no pizza for us! Learn more about Pluto!
There is a theory that the earth was out at that greater distance and some large object smashed into it breaking it into smaller pieces, the asteroid belt. The now smaller earth was shoved inward to its present position. It’s a good thing since our present distance from the sun is the temperature sweet spot for life to exist.
Out past Pluto is another field of icy bodies called the Kuiper Belt. More than 2,000 icy bodies have been found hiding there. In fact, in 2005, a large Kuiper Belt body was identified, larger than Pluto, called Xena. If Pluto is the ninth planet then Xena is the tenth. Or Ceres is the tenth. So they are not planets. They are dwarf planets.
So after 76 years of having the exalted title of planet, Pluto lost its title in a political skirmish of all or none. So its none! So goodbye to the runt, Pluto!