One problem that faced scientists when Pluto was discovered was what it should be called. All of the planets in the Solar System (apart from Earth) are named after Roman/Greek mythological gods. This is because the Romans and Greeks believed the objects in the night sky to be their gods. The only five planets to be known about in Roman and Greek times were Mercury, Venus, Mars, Jupiter and Saturn, seeing as these were the only five planets visible from Earth and bright enough to be considered "gods". These planets still have the names that the Romans and Greeks gave them when they believed them to be their gods. However, planets discovered long after the Roman and Greek ages had ended still continued the tradition of having mythological names.
Here is a quick run-down of the planets, where their names come from and what they mean. Mercury was the Roman god of commerce, travel and industry, equivalent to the Greek god Hermes (the planet's other name), the messenger of the Gods. It was named Mercury perhaps because of the speed at which the planet orbits the Sun. Venus is named after the Roman goddess of love and beauty because it is the brightest and most beautiful object in the night sky after the Sun and the Moon. As its opposite, Mars is named after the Roman god of War, because of its blood red colour. Jupiter, the largest planet in the Solar System, is named after Jove, the Roman King of the gods. Its Greek name is Zeus. The is simply because of its size in comparison to the rest of the planets in the Solar System. Zeus was the son of Cronus, the Greek name for the next planet in the Solar System, Saturn. Saturn was the Roman god of agriculture, ironic because the planet has no surface to farm. Uranus, discovered in the 18th Century, was named after the Greek ruler of the Heavens and, in mythology, is the father of Saturn. Neptune is the god of the Sea, possibly named because of its sea-blue colour (which is caused by methane in the planet's atmosphere).
Finally, there is Pluto. Although it is now not recognised as a planet, for the 76 years after its discovery, it was known as the ninth planet in the Solar System. But, because it was discovered relatively recently, and was not observed by the Romans and Greeks as one of their gods, choosing its name had to be left to scientists. As with Uranus and Neptune, two planets which were also not observed by the Romans or Greeks, the many suggestions of names for the new planet were still largely related to ancient mythology. The earliest suggestions were Atlas, Zymal, Artemis, Perseus, Vulcan, Tantalus, Idana, Cronus (as mentioned earlier, Cronus is the Greek name for Saturn). Other suggestions were Minerva, Osiris, Bacchus, Apollo and Erubus. The widow of Percival Lowell suggested Zeus and then Constance, whereas other people believed that the new object should be named Lowell, after the man who first searched for the ninth planet (which he referred to as Planet X). He died in 1916, despite recording images of Pluto (but not recognising them as images of his planet). The planet was properly discovered by Clyde Tombaugh in 1930. Suggestions for names by staff at the Flagstaff Observatory, where the planet was discovered, included the previously suggested Cronus and Minerva, and the planet's eventual name Pluto. The suggestion of the name Pluto did not come from any scientists or astronomers. Instead, it was suggested by an eleven year old schoolgirl, living in Oxford, England, called Venetia Burney!
What was the ninth planet in the Solar System now has the name Pluto because Pluto was the Roman god of the Underworld. This is because Pluto, being so far away from the Sun, receives very little light. It is also believed that Pluto was chosen for the planet's name because the first two letters "P" and "L" are the initials of Percival Lowell. Coincidentally, in the same year as the planet Pluto was discovered, Mickey Mouse's dog made his first appearance in a Disney cartoon ("The Chain Gang"). Later that year, he appeared as Minnie Mouse's dog, Rover, and, in the following year, received his name, Pluto, in Mickey Mouse's cartoon "The Moose Hunt"!
The story of how Pluto got its name is told in a short documentary film called "Naming Pluto". This film includes an interview with Venetia Burney, the schoolgirl who came up with Pluto's name. Find out more here!