All of the planets, except for Earth, were named after Greek and Roman gods and godesses. Jupiter, Saturn, Mars, Venus and Mercury were given their names thousands of years ago. The other planets were not discovered until much later, when telescopes were invented. The tradition of naming the planets after Greek and Roman gods and goddesses was carried on for the last three planet discovered as well. Mercury was named after the Roman god of travel. Venus was named after the Roman goddess of love and beauty. Mars was the Roman god of War. Jupiter was the king of the Roman gods, and Saturn was the Roman god of agriculture. Uranus was named after an ancient Greek king of the gods. Neptune was the Roman god of the Sea and Pluto was the Roman god of the underworld. The name Earth is an English/German name which simply means the ground. (from
http://coolcosmos.ipac.caltech.edu/cosmic_kids/AskKids/planet_names.shtml)
You may have heard that a new "planet" has been found, and has been tentatively named "Sedna", after the Inuit goddess of the sea. Check out this article for more on this -- and also about the time when an astronomer tried (and failed) to break from the god/goddess naming trend and name the planet that eventually became Uranus after King George III of England. Check out this link for the whole story:
http://news.bbc.co.uk/2/hi/uk_news/magazine/3515658.stm cheers,
Sarah