Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).
Earth is the English word meaning ground. Actually nobody named our planet earth. It was Nicolas Copernicus who was astronomer showed that the ground on which we are standing is one of the planet of our solar system. People start calling this ground as earth. Names of all planets including Sun and moon are simply words or folk names which were originated when human languages started.