|Discovery Date:||February 18, 1930|
Pluto is today considered a Dwarf Planet. It has 5 known moons called Charon, Hydra, Kerberos, Nix and Styx. The mass of Pluto is smaller than our moon. Pluto is very far from the Sun and very cold at 229°C. Pluto takes 246.04 Earth Years to orbit the sun. 
When Pluto was last in the same position relative to the sun where it is now Copernicus, Galileo and Kepler had shown that the The Earth orbits the sun and Isaac Newton had discovered Gravity. Scientists didn't know yet whether the sun is the centre of the Solar System only or the centre of the Universe. Astronomers speculated that the sun might be a star but no one could prove it yet.  Then what became the United States of America was still a colony and the American Revolution hadn't started yet. A great deal happened on earth while Pluto made just one orbit.
Teh 20th century
During the 20th Century astronomers and the general public called Pluto a planet. Then it was discovered that there are many similar small bodies in the Solar System. Scientists held a convention in 2006  and decided that Pluto and several others would in future be called Dwarf Planets. Plenty of people were angered by this and still are, which is a great example of making a big deal over nothing.  
When it was a Planet, it was the smallest in the Solar System.
A poem about Pluto
"There once was a planet named Pluto
Whose title got sent to Maputo
So poor Pluto got sad,
And the nerds they got mad,
Confusion? We say "Benvenuto!"- Poem originally at RationalWiki
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- Bye Bye Planet Pluto Video about Pluto
- The Nine Planets - Pluto Video teaching about Pluto
- Why Isn't Pluto a Planet Any More? Cool video