
Pluto is the ninth and farthest planet from the Sun, and also the smallest, and is smaller than the seven largest moons in the solar system, including Earth's Moon. It used to be the ninth planet in the solar system when Baby Galileo was released and is therefore a planet in the Baby Einstein canon, but was reclassified as a dwarf planet in August 2006. It was discovered in 1930 by American astronomer Clyde Tombaugh, and it was named by an 11-year-old schoolgirl from Oxford, England named Venetia Burney, naming the planet after the Roman God of the underworld. Disney mascot Mickey Mouse even named his pet dog after the then newly discovered planet. One day on Pluto lasts almost a week on Earth and one year on Pluto lasts 248 Earth years. Pluto is one of the coldest bodies in the solar system, as it receives 1600 times less sunlight than Earth does. Its surface is very icy and rocky. It has an unusual orbit that sometimes gets closer to the Sun than Neptune. Pluto has five moons, the largest being Charon, with the other four being Nix, Hydra, Kerberos and Styx. Charon has its own significant gravity that affects Pluto and makes them orbit like a binary star system. The only spacecraft to ever visit Pluto was New Horizons in 2015. According to the International Astronomical Union, in order to be a planet, Pluto must orbit the Sun, be big enough for gravity to shape it into a ball, and clear the neighborhood around its orbit. Rule number three is what disqualifies Pluto; its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times.