Various groups and people named the planets. The Romans named the five planets that are visible to the unaided eye: Mercury, Venus, Mars, Jupiter and Saturn. The names are based on the planets' appearances and movements. The Roman names were adopted by European languages and cultures, and they eventually became standard.
Uranus and Neptune were discovered before there was an established method for naming plants. They were called by several names until one eventually became standard. For example, William Herschel discovered Uranus and wanted to name it after King George II, but Johann Bode suggested the name Uranus, to match the naming conventions of the previous planets. It wasn't until 1850 that Uranus was commonly used as the name.
No one person named Neptune; rather, the two astronomers who discovered it accepted the name after it was proposed to them. Since then, the name Neptune has become the standard name.
Clyde Tombaugh discovered planet Pluto in 1930; a child from Oxford, England suggested the name. Astronomers recommended the name to the observatory staff.
Today, planets and all other celestial objects are named by the International Astronomical Union. When scientists discover new planetary objects or features, they may suggest names to the IAU. The IAU either accepts the proposed name or suggests a different one. Because experts consider the finding of new planets unlikely, the group concentrates on naming moons, planetary features and comets.